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The structure of high-redshift clusters from NIKA Sunyaev-Zel'dovich observations

Presenter: Rémi Adam
Authors: R. Adam, B. Comis, J.-F. Macías-Pérez, et al.


         Recent Sunyaev-Zel'dovich (SZ) observations (e.g. Planck, ACT, SPT) have proved to be competitive and complementary to traditional observations of galaxy clusters for cosmology. Nevertheless, these surveys are limited by angular resolution and systematic effects due to the complex physics occurring in the intra-cluster medium (ICM). In order to better address the physics at play, dedicated follow-ups with much higher angular resolution are therefore necessary.With its dual-band capabilities at 150 and 250 GHz, and an angular resolution of 18 and 12 arcsec, the NIKA camera is an ideal instrument for such observations. It is based on newly developed Kinetic Inductance Detectors (KIDs) and operates at the IRAM 30-m telescope (Pico Veleta, Spain). NIKA is the prototype of the larger future camera NIKA2, it has been used to map the ICM of a representative set of clusters at z>0.45, as part of a pilot study for future NIKA2 large programs. After briefly introducing the specificities of NIKA, I will present the status of SZ observations performed at the IRAM 30-m telescope, including the most recent results.



Planck Cosmology with clusters

Presenter: Nabila Aghanim


         The Planck survey has delivered the largest all sky cluster catalogue selected through their Sunyaev-Zeldovich (SZ) signal. This has permitted discover hundreds of new galaxy clusters, to populate a unique window of the most massive (M>5 10^14) highest redshift (z>0.5); and to derive stringent cosmological constraints on a set of parameters. I will present the latest results derived from the analysis of SZ number counts from Planck and discuss the intrinsic limitations set by the issues of observable-to-mass relation, in particular the Y-M relation.



The Spectacular Merger Event in A3411: Shock Fronts and Radio Relics

Presenter: Felipe Andrade-Santos
Authors: Felipe Andrade-Santos, Reinout van Weeren, Christine Jones, Georgiana Ogrean, William Forman, Stephen Murray, Kevin Fogarty


         The study of galaxy cluster merger events is of majorastrophysical interest as they have a profound and long-lastingimpact on the thermodynamic evolution of the ICM. Observed aspart of our large Chandra XVP program on the Planck ESZsample, we discovered that the cluster A3411 is undergoing aspectacular merger event. Radio observations also reveal large-scalediffuse emission, suggesting the presence of shocks andturbulence in the ICM. Most interestingly, in the Chandraobservations we indeed find evidence of a brightnessdiscontinuity, roughly at the location of the radio emission,corresponding to a low-Mach number shock. This suggests thateven low-Mack number shocks could be responsible for theacceleration of particles to relativistic energies and makes A3411an ideal laboratory to study this poorly understood process.



Extending Robust Weak Lensing Masses to High Redshift

Presenter: Douglas Applegate
Authors: Douglas Applegate, Tim Schrabback, and the SPT-Lensing Group


         Weak gravitational lensing measurements are essential for calibrating multi-wavelength mass proxies, thereby allowing us to robustly extract cosmological constraints from cluster surveys. Independent lensing measurements of cluster masses now agree to better than 10%, at least for clusters at z < 0.7. However, it is unclear whether these existing methods scale to higher redshifts. In addition, Sunyaev Zel’dovich (SZ) surveys are detecting large numbers of massive clusters at z > 0.7, a regime where the evolution of mass proxies has not been thoroughly tested. SZ surveys, especially when combined with existing X-ray surveys, should strengthen constraints on beyond-LambdaCDM physics, but only if we can calibrate mass proxies at these higher redshifts to better than 15%.In this talk, I will present new results on calibrating multi-wavelength mass proxies at high redshift with weak lensing. For this project, we have measured weak lensing masses for 13 clusters selected from the South Pole Telescope SZ survey and imaged with the VLT and Hubble. This allows us to calibrate mass proxies to a statistics-limited 15%. I will highlight how we have used large n-body simulations and the Hubble treasury program CANDELS to control systematic uncertainties. Our results serve as a pathfinder for an expanded sample of over 50 clusters at z > 0.7 already imaged with HST. With this expanded sample, we can expect to improve the calibration of mass proxies at z > 0.7 to better than 10%.

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Bartalucci - Poster

Towards a combined analysis of high redshift clusters with XMM-Newton and Chandra

Presenter: Iacopo Bartalucci
Authors: Iacopo Bartalucci


         We present a comparison of radial density profiles of clusters at various z up to z~1 clusters, observed with Chandra and XMM-Newton. We apply the same de- projection technique of surface brightness profile, taking in account both projection and PSF (for XMM), in order to retrieve the radial density profile. The accuracy of the XMM-PSF correction is discussed and prospect for combined analysis.


Bharadwaj - Poster

The L-T scaling relation on the galaxy group regime: Impact of ICM cooling, AGN feedback and selection effects

Presenter: Vijaysarathy Bharadwaj
Authors: V. Bharadwaj, T. H. Reiprich, L. Lovisari, G. Schellenberger, H. J. Eckmiller, R. Mittal, H. Israel

         X-ray scaling relations constructed from existing high quality X-ray would be an important asset for upcoming all-sky surveys (e.g. eROSITA) where the data would not be sufficient to constrain the physical properties directly. These scaling relations are however affected by phenomena such as the cool gas in the cores of clusters, and/or potentially AGN feedback. Selection effects also contribute to a biased determination of the scaling relations. It is also worth investigating whether cluster scaling relations are valid on the low-mass galaxy group regime.With a sample of 26 galaxy groups, we endeavored to construct the bolometric L-T relation on the group regime. We fit the relation for five cases; the complete sample, sub-samples which factor the presence/absence of a strong cool-core, and the presence/absence of a central radio source. To perform bias corrections we undertook simulations and compared the slopes, normalisations, and intrinsic scatters to the bias-corrected relation on the cluster regime. We finally provide a broad picture on extending the L-T relation from the cluster to the group regime.



Results from the 2500d SPT-SZ Survey

Presenter: Lindsey Bleem
Authors: SPT Collaboration


         In Fall 2011 the South Pole Telescope (SPT) completed the SPT-SZ survey, a three band mm-wavelength survey of ~2500-square-degrees of the southern sky. One of the primary objectives of this survey was the creation of a mass-limited sample of galaxy clusters selected via the Sunyaev- Zel’dovich (SZ) Effect. The resultant cluster sample consists of ~500 galaxy clusters (>80% new discoveries) with a median mass of M500~3.5x10^14 Msun/h, a median redshift of z=0.55, and a maximum spectroscopic redshift to date of 1.47. The high-redshift portion of this cluster sample will be a tremendous resource for understanding the changes in star formation rates, galaxy populations, the intracluster medium, and AGN activity during the critical period when massive clusters are forming; to date, 36 clusters have photometric redshifts z < 1 and 11 of these have been spectroscopically confirmed. In this talk I will describe the cluster sample and associated cosmological constraints as well as our explorations of the highest redshift SPT systems. I will also preview upcoming cluster results from SPTpol -the current receiver deployed on the SPT- and projections for the SPT-3G survey, which will commence in 2016 and is expected to identify more than 1000 z < 1clusters.



About the impact of baryons on the cluster mass function: fitting formula and cosmological implications

Presenter: Sebastian Bocquet
Authors: Bocquet, Saro, Dolag, Mohr


         We calibrate the cluster mass function accounting for halo baryons using the hydrodynamical Magneticum simulations. These are well suited because of their large cosmological volumes of up to 1 Gpc^3. Baryonic effects globally decrease the cluster masses, which, at given mass, results in a decrease of the number density of galaxy clusters.We examine the potential cosmological biases due to differences between our hydrodynamic and our own dark matter-only functions, and common dark matter mass functions from the literature. We perform cosmological analyses of two simulated cluster surveys: (1) an approximately SPT-like sample that mostly contains high-mass systems, and for which we find that the impact of baryons on the cluster mass function effectively vanishes. (2) an approximately eROSITA-like sample, which extends to much lower cluster masses. In this case, we find that neglecting the baryonic impact leads to an underestimate of Om of about 0.03, a shift that is significant given the overall expected uncertainty of ?Om ? 0.012.Also, care must be taken not to introduce incorrect cosmological sensitivity by choosing inadequate parametrizations of the mass function. The comparison with fitting functions from the literature indeed suggests that further studies are needed in order to properly determine the systematic uncertainties on the mass functions, which will become crucially important for progress in cluster cosmology.



Dark Matter and First Galaxies Light Up

Presenter: Marusa Bradac


         The Bullet Cluster has been the subject of intense research in thelast few years. This system is remarkably well-suited to addressingoutstanding issues in both cosmology and fundamental physics. I willpresent our newest measurements of the composition of this system, anddescribe the limits that can be placed on the intrinsic properties ofdark matter particles. In this talk I will also present new resultsfrom a much larger sample of Bullet-like cluster, MACSJ0025-1222,A520, and DLSCL J0916.2+2951 among others.In the second part of my talk I will describe our recent endeavorswith Hubble Space Telescope that enabled us to detect galaxies at thetime when the Universe was only a few procent of its current age. Theyare likely beacons of the epoch of reionization, which marked the endof the so-called ``Dark Ages'' and signified the transformation of theuniverse from opaque to transparent. Clusters of galaxies, when usedas cosmic telescopes, can greatly simplify the task of studying andfinding these galaxies. The main prerequisite for such studies areaccurate mass maps which are needed to convert observed (lensed) intointrinsic (unlensed) quantities and properly account for observedvolume. My team is producing these maps for the Frontier Fieldinitiative and I will describe the ongoing efforts.



Merging Clusters, Particle Acceleration and Dark Matter

Presenter: Marcus Bruggen


         I present some recent research highlights of the Merging Clusters Collaboration (MCC). The past two years have seen tremendous progress in understanding (i) the acceleration of electrons in low-M, high beta shocks, (ii) constraints on the cross section of self-interacting Dark Matter and (iii) star formation and galaxy evolution in merging clusters. I will review a very broad observational and theoretical campaign to study these topics.



Physics of turbulent acceleration in galaxy clusters

Presenter: Gianfranco Brunetti
Authors: G. Brunetti


         Giant radio halos suggest that relativistic particles can be (re)accelerated in galaxy clusters by turbulence generated during cluster-cluster mergers. The efficiency of turbulent acceleration depends on fundamental aspects of the (micro-)physics of the underlying ICM that are still poorly constrained. In this respect radio halos can be seen as complementary probes of the complex hierarchy of processes that transports energy from large-scale motions to smaller scales.I will discuss these aspects of turbulent acceleration and the importance of future observations with the next generation of radio and high-energy telescopes.


Brunetti - Poster

Constraining magnetic fields in radio relics: the case of Abell 521

Presenter: Gianfranco Brunetti
Authors: G. Brunetti

         Radio relics are elongated, peripheral and polarised cluster-scale radio sources that are generally found in merging systems. According to the current leading scenario, relics trace gigantic cosmological shock waves that cross the intra-cluster-medium (ICM) where particles are accelerated. Despite their apparent similarity with supernova remnants (SNRs), where relativistic particles (cosmic rays (CRs)) are in fact accelerated by diffusive shock acceleration (DSA), the details of the mechanisms that are responsible for the (re)acceleration of relativistic electrons in radio relics and for the amplification of magnetic fields

         Markevitch et al. (2005) proposed a method to constrain the magnetic field strength in radio relics independently from the details of the mechanisms that are responsible for the acceleration of the emitting electrons at cluster shocks. This method is based on the measure of the thickness of radio relics at different observing frequencies under the assumptions that relics are powered by relativistic electrons (re)accelerated in the shock region that age downstream of the shock. Although this method is -in principle- straightforward, in practice it is affected by several uncertainties, including projection effects and the role of reacceleration/diffusion in the downstream region. In this poster we focus on the radio relics in Abell 521. The relic in A 521 is imaged at several frequencies (Giacintucci et al. 2008), whereas X-ray observations (Bourdin et al. 2013) have measured the main properties of the underlying, including its Mach number. Combining radio images and X-ray measurements with a simple geometrical model we derive constraints on the magnetic field strength in the radio relic and upstream of the shock.



Constraints on the keV Scale Dark Matter Candidates

Presenter: Esra Bulbul
Authors: Maxim Markevitch, Adam Foster, Randall Smith, Mike Loewenstein, and Scott Randall

         Observations of astronomical objects have the potential to reveal a decay or annihilation signal from dark matter. A warm dark matter candidate, sterile neutrino at keV scale, may oscillate to an active neutrino by emitting an X-ray photon. Another viable candidate is a self-interacting exciting dark matter, which could produce an X-ray line. I will discuss current constraints on keV sale dark matter models with a particular focus on the recent detection of the unidentified 3.55 keV emission line in galaxy clusters, dwarf spheroidals, and galaxies.

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Cold Clouds in Cool Cores

Presenter: Becky Canning


         Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of cool (~10,000 K) and cold (~100 K) gas. This gas can be distributed in filamentary structures, which are long lived and are typically not forming stars. Two important questions are; to what temperature do these reservoirs cool and if cold gas is abundant what is the cause of the low star formation efficiency? Understanding the heating and excitation mechanisms in the cold gas can lend important insight into these questions. I’ll show current observations of extended cold gas in massive galaxies and discuss models of the theoretical properties of dense, cold clouds embedded in hot group/cluster atmospheres. I will present predictions for further observations which can test these models.



Kinetic Simulations of Particle Acceleration at Astrophysical Shocks

Presenter: Damiano Caprioli
Authors: Damiano Caprioli


         Particle-in-cell (PIC) simulations are providing us unprecedented insights into the microphysics of collisionless shocks, also attesting to their ability to accelerate particles and generate magnetic fields. I present state-of-the-art full PIC and hybrid (kinetic ions - fluid electrons) simulations of non-relativistic shocks, discussing under which conditions (shock strength and inclination) ions and electrons are injected and energized via diffusive shock acceleration. I also outline how the initial magnetic field is amplified by plasma instabilities induced by energetic particles, which has both observational and theoretical implications. Finally, I discuss the relevance of these findings for non-thermal emission and cosmic ray acceleration in astrophysical sources, such as supernova remnants and galaxy clusters.



On the connection between giant radio halos and cluster mergers

Presenter: Rossella Cassano

         Giant radio halos are diffuse, Mpc-sized, synchrotron radio sources with steep radio spectra that are observed in the central regions of a fraction of galaxy clusters. They probe the energy content and properties of relativistic particles and magnetic fields in galaxy clusters and their imprint on cluster formation and evolution. There are growing observational evidences in favor of a connection between radio halos and cluster mergers. This connection suggests that radio halos are generated as a result of the dissipation of gravitational energy during the hierarchical sequence of mergers that leads to the formation of clusters themselves. I will review current results on the formation rate of radio halos in galaxy custers and on their connection with cluster mergers, discussing the impact of these results on our current understanding of the origin of radio halos in galaxy clusters.



AGN feedback and perturbed cluster cores.

Presenter: Eugene Churazov
Authors: E.Churazov et al.


         Much of our current knowledge on the AGN feedback in clusters comes from the analysis of perturbed X-ray images. We discuss additional information on the feedback process, recorded in spectral variations, associated with these perturbations. We argue that statistical analysis of many small fluctuations in different X-ray energy bands provides a convenient way of characterizing the nature of perturbations.



Commensal Cluster Astrophysics with the VLA Low Band Ionospheric and Transient Experiment (VLITE)

Presenter: Tracy Clarke
Authors: T. E. Clarke, N. E. Kassim, J. Helmboldt, W. Peters, T. Mroczkowski, S. Giacintucci, E. Polisensky


         Radio observations toward galaxy clusters allow us to tracethe AGN-driver outflows in jets and lobes that interact with thecluster medium as well as the diffuse synchrotron component (halos and relics) driven by shocks and turbulence from merger activity. Radioobservations below 1 GHz (so called Low Band) have been essential toprobing multiple generations of AGN feedback as well as the particleacceleration associated with shocks and turbulence from clustermergers. I will present a new commensal radio experiment running onthe NRAO Very Large Array which provides 'free' Low Band radio dataduring nearly all PI-driven observing programs. I will highlight somepossible uses of this data for enhanced science of PI-driven programsand well as general cluster astrophysics.


Connor - Poster

Properties of CLASH Cluster Galaxies

Presenter: Thomas Connor
Authors: Thomas Connor, Megan Donahue, Daniel Kelson, Marc Postman, CLASH Team


         CLASH has HST images of 25 massive galaxy clusters using 16 filters, which provides an unprecedented data set for exploring the properties of cluster galaxies. Accurate photometric measurements of the member galaxies require disentangling the emission of spatially coincident objects, a feature particularly common around the cluster center. We present measurements of galaxy photometry around each CLASH cluster, along with photometric redshifts and cluster membership determinations. From this we investigate the effects of metallicity and evolution on the colors of cluster galaxies.


Crosby - Poster

Simulating Realistic Clusters With Galaxy Particles

Presenter: Brian Crosby
Authors: Brian Crosby, Brian O'Shea

         Cosmological simulations of galaxy clusters have made great strides in reproducing the observed bulk properties of the ICM, but the cluster galaxy populations and properties of the ICM in cluster cores have failed to match observations. A key reason for this discrepancy is the “star particle” method with which stellar populations are represented in cluster simulations. This star particle method is tailored to galactic-scale simulations, and while it has enjoyed success in that realm, the physical processes encapsulated within their models for formation and feedback are not appropriate for physical scales and resolution constraints of cosmological galaxy cluster simulations. I will present early results from simulations utilizing a new “galaxy particle” method specifically designed to be applicable the physical scales of a cluster environment while operating efficiently within the resolution constraints of modern supercomputer simulations. The galaxy particle method treats galaxies as individual particles composed of an ensemble of semi-analytic models governing internal galactic processes such as star formation and feedback. The relevant physical scales of these processes are fundamentally smaller than the resolution limit of galaxy cluster simulations, and physically motivated semi-analytic models allow the galaxy particle method to avoid introducing spurious, unphysical, numerical effects. Additionally, galaxy particles have physical extent, allowing for the interaction between galaxies and the ICM to be modeled in a much more realistic manner than has been previously possible. In this poster I will present a comparison between clusters simulated with the galaxy particle method and those simulated with a standard star particle method. I will discuss differences in the number, color, and morphology of the galaxy populations, as well as the thermal and chemical state of the ICM in cluster cores, highlighting improvements in our ability to reproduce galaxy cluster observations with the galaxy particle method.


Cuciti - Poster

Radio Halos in mass-selected galaxy clusters: testing theoretical models

Presenter: Virginia Cuciti
Authors: V. Cuciti, R. Cassano, G. Brunetti, D. Dallacasa, R. Kale


         A fraction of merging galaxy clusters host Mpc-scale radio halos (RH), generated by ultrarelativistic electrons in the magnetized intra cluster medium (ICM). An unbiased analysis of the statistical properties of RHs is crucial to determine their origin and evolution. The recent advent of cluster surveys via the SZ effect has enabled the construction of almost mass-selcted cluster samples, due to the tight relation between the total SZ signal and the mass. We selected from the Planck SZ catalogue clusters with M>6x10^{14}Msun in the redshift range z=0.08-0.33 and we searched for the presence of RHs in these clusters using the NVSS for z<0.2 and the GMRT RH Survey and its extension (EGRHS) for 0.2 < z < 0.33. We found a statistically significant evidence (at 3.26 sigma) of a drop of the fraction of clusters with RHs toward low massive systems and show that this drop is in linewith the predictions of the re-acceleration scenario. By means of Chandra X-ray data we studied the cluster dynamical status and we confirmed that RHs are preferentially found in merging systems, thus supporting the idea that mergers play a fundamental role in the generation of RHs.



Sunyaev-Zel’dovich Scaling Relations Measured at 140 GHz with Bolocam

Presenter: Nicole Czakon
Authors: N. G. Czakon, J. Sayers, A. Mantz, S. R. Golwala, T.P. Downes, P.M. Koch, K.-Y. Lin, S. M. Molnar, L. A. Moustakas, T. Mroczkowski, E. Pierpaoli, J. A. Shitanishi, S. Siegel, and K. Umetsu


         The Sunyaev-Zel’dovich effect (SZE) is a powerful tool to study galaxy clusters out to large radii and to detect clusters at high redshifts. To first order, clusters or self-similar and one can link the SZE signal to a cluster’s physical properties by assuming a spherical distribution of matter in hydrostatic equilibrium. The SZE signal, however, will be affected by any astrophysical process that contributes non-thermal pressure support or if the cluster has non-spherical morphology. We have measured the SZE signal of 45 massive clusters using Bolocam at 140 GHz. After measuring the scaling relations of the SZE signal with total cluster mass, we find our clusters to be approximately 5-sigma shallower than the self-similar HSE prediction--a result that is in tension with most other SZE scaling relations studies. To confirm our measurements, we have implemented a series of tests to see whether, among others, sample selection, redshift, degree of disturbance, or alternative mass proxies might affect our measurements. We believe our results to be robust to the extent to which we are able to constrain the cluster properties with current observations. If confirmed, this would have a major impact on our understanding of galaxy clusters and cluster cosmology.

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Dai - Poster

The Swift AGN and Cluster Survey

Presenter: Xinyu Dai
Authors: X. Dai (OU), R. D. Griffin (OU), J. M. Nugent (OU), C. S. Kochanek (OSU), J. N. Bregman (UM)

         The Swift AGN and Cluster Survey (SACS) uses 125 square degrees of Swift XRT serendipitous fields with variable depths surrounding gamma-ray bursts to provide a medium depth (4e-15erg/cm^2/s) and area survey filling the gap between deep, narrow Chandra/XMM-Newton surveys and wide, shallow ROSAT surveys. Here we present a catalog of 22,563 point sources and 442 extended sources and examine the number counts of the AGN and galaxy cluster populations. SACS provides excellent constraints on the AGN and cluster number counts, and the cluster number counts span a much larger continuous flux range than previous surveys. Using the public SDSS data, we have measured photometric redshifts for the majority of z<0.5 Swift clusters that fall in the SDSS regions. Deep optical or IR follow-up observations of this cluster sample will significantly increase the number of higher redshift (z < 0.5) X-ray-selected clusters.


Datta - Poster

Understanding Merger Activities in Galaxy Clusters Using High Fidelity X-ray Temperature Maps and Radio Observations

Presenter: Abhirup Datta
Authors: Dr. Abhirup Datta, Prof. Jack O. Burns, Dr. Samuel W. Skillman & David E. Schenck

         In the hierarchical structure formation framework, clusters of galaxies are the largest virialized objects in the universe and are ideal laboratories to study astrophysical plasma processes. Clusters are assembled through large and small merger activities which are among the most energetic events in the universe. During mergers cosmological shocks are driven into the intracluster medium (ICM). These shocks heat the ICM which is then detected in the soft X-ray regime through its thermal emission. In addition, these shocks accelerate non-thermal electrons and protons to relativistic speeds. The relativistic electrons have relatively short lifetimes (108 years) and emit synchrotron radiation which is then detected at the radio wavelengths. Steep spectrum radio relics and halos are now recognized as clear signposts of recent mergers. Direct evidence for cluster mergers has also been found from the disturbed X-ray surface brightness of the ICM. X-ray temperatures are very sensitive probes of recent merger or accretion events from the large-scale cosmic web. The understanding of the cluster mergers can be enhanced by combining the X-ray (high fidelity temperature maps) and radio (spectral index maps) observations. In this talk, we present our recent work on two galaxy clusters: Abell 3667 (non cool-core cluster with a cold front and hosting a double radio-relic) and Abell 85 (cool-core cluster hosting a single radio-relic). The high-fidelity X-ray temperature maps allowed us to study the X-ray shocks in the cluster using a new two-dimensional shock-finding algorithm. We have also compared the X-ray and radio observations with state-of-the-art adaptive mesh refinement MHD cosmological simulations using Enzo.



Dark Matter in the Hubble Frontier Fields Clusters

Presenter: Jose M Diego
Authors: Jose M Diego et al.


         Using a robust free-form method for lensing reconstruction we identify multiple new images in deep images of clusters from the Hubble Frontier Fields (HFF) program. We use the newly identified images together with other systems from the literature to constrain the mass distribution in the central region of the HFF galaxy clusters. We present results for 4 clusters, A2744, MACS0416, MACS0717 and MACS1149. Our results show interesting correlations between the mass distribution and X-ray plasma indicating that the lensing data may be sensitive to the mass of the plasma. We also find very shallow profiles in these clusters. Such shallow profiles are difficult to reproduce with standard CDM N-body simulations while they are more easily predicted in simulations with self-interacting dark matter or viscosity.



On the (non-)universality of cluster density profiles and concentrations

Presenter: Benedikt Diemer
Authors: Benedikt Diemer, Andrey Kravtsov


         The density profiles of cluster halos are an essential input for the interpretation of numerous observations, such as weak and strong lensing. These profiles are commonly modeled using the NFW form, assuming that they depend on only two parameters: mass and concentration. In this talk, I will show that the outer halo density profiles depend on an additional parameter, the mass accretion rate, and present an accurate new fitting formula that takes this dependence into account. Similarly, I will present a new, universal model for halo concentrations that is based on a previously unknown dependence of concentration on the local slope of the matter power spectrum. This model is in excellent agreement both with simulations and with the recent observations of the CLASH cluster survey. I will discuss the implications of the model in the context of the recent debate about a possible upturn in the c-M relation at high masses, and introduce a public code to compute the model predictions.



Magnetic Fields and Thermal Conduction in Cosmological Simulations

Presenter: Klaus Dolag


         Astronomical surveys and instruments (current and upcomming) like Planck, SPT, PanStars, DES, Euclid, LOFAR, eRosita and many more will need a theoretical counterpart in form of simulations which follow the formation of cosmological structures in so far unaccomplished detail, taking into account enough physical processes to allow a self consistent comparison to observations at multiple wavelength and throughout the entire epoch of structure formation. I will report on preliminary results from a recent simulation campaign, where we followed the formation of cosmological structures in so far unaccomplished detail, performing a large set of cosmological, hydrodynamical simulations covering up to Gpc^3 volumes, taking into account various physical processes (star-formation, chemical enrichment, AGN feedback, magnetic fields, transport processes) to allow a self consistent comparison to observations at multiple wavelength. I will focus on the results of a subset of these simulations which are especially following the evolution of magnetic fields and its role on transport processes.



Cluster Science Highlights from the CLASH Multi Cycle Treasury Program

Presenter: Megan Donahue


         I will review some of the cluster science results from the Hubble Space Telescope CLASH MCT program, including a summary of the mass profiles, X-ray gas profiles. A comparison of the X-ray and lensing mass profiles of CLASH yield some interesting insights on some of the early tension between Planck cosmological parameters and Planck cluster counts. I will briefly describe some of the CLASH cluster galaxy studies underway, and plans for future work.

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Cold gas in cluster cores

Presenter: Alastair Edge
Authors: Alastair Edge


         The presence of cold molecular gas in the cores of clusters of galaxies appears to be strongly connected to the properties of the ICM close to the BCG. The levels of AGN and star formation activity in the BCG are well correlated to the amount of gas suggesting it is the fuel for both. I will review the current results on the properties of the cold gas in cluster and group cores and prospects for ALMA to map the dynamics of this gas directly.


Egan - Poster

ICM Plasma Homogeneity across Multiple Clusters

Presenter: Hilary Egan
Authors: Hilary Egan, Brian O'Shea, Eric Hallman, Jack Burns

         The intracluster medium is a magnetized fluid with very large mean free paths. Although this validates a fluid approach at large scales, at scales below the resolution of typical simulations, magnetic field driven instabilities can grow. These instabilities could propagate to larger scales, having strong effects on the viscosity, conductivity, and turbulence in the ICM. One way to circumvent this resolution driven problem is to create sub-grid models of the instabilities using high resolution simulations of small volumes; however, the plasma properties of the high resolution simulation must be broadly applicable in order for the sub-grid models to be appropriate. Here I present an analysis of plasma properties across ten clusters with a variety of sizes and dynamical states simulated at high resolution using ideal MHD Enzo. Using multiple metrics including coherence length, Debeye length, and anisotropy parameter, we find that each measure shows strong similarity across clusters. By showing that these properties are homogeneous across clusters, we find that sub-grid models are both needed and scientifically justifiable.



Galaxy Mergers in the Nearby Universe

Presenter: Sara Ellison


         Mergers and interactions are a fundamental process in the life cycle of galaxies. In this talk, I will present the results from a multi-wavelength analysis of galaxy mergers in the SDSS, that provides a panoramic investigation of the role of mergers on star formation, gas consumption, AGN triggering and ISM chemistry. The sample can also be used to study mergers separately in the group, cluster and field environments, in order to evaluate the relative importance of large and small scale processes. I will also present the results from a large suite of hydrodynamic merger simulations, which provide a powerful tool for the interpretation of the observational results.


Emig - Poster

The Toothbrush Cluster at 60 MHz with LOFAR

Presenter: Kimberly Emig
Authors: K. Emig, F. de Gasperin, R. van Weeren, M. Bruggen


         Low-frequency observations with telescopes like LOFAR allow us to explore radio features which trace the interaction histories of galaxy clusters. I used LOFAR LBA to image the low-frequency radio emission (10–90 MHz) from the merging galaxy cluster 1RXS J0603.3+4214 (z = 0.225; "Toothbrush" cluster). These observations revealed a Mpc-size radio relic, tracing shock waves propagating through the intra-cluster medium, and a radio halo, a likely signature of merger driven turbulence. To analyze these observations, I developed an algorithm to enhance telescope performances by exploiting the time-correlation of the main systematics affecting the instrument. With this approach, I produced some of the lowest noise images to date at these frequencies.


Erler - Poster

Evidence for a shock in the Coma relic from Planck SZE data

Presenter: Jens Erler


         Radio relics are Mpc-scale diffuse synchrotron sources found in galaxy cluster outskirts. They are believed to be associated with large-scale shocks propagating through the intra-cluster medium, although the connection between radio relics and the cluster merger shocks is not yet proven conclusively. We present a first tentative detection of a pressure jump in the well-known relic of the Coma cluster through Sunyaev-Zel'dovich (SZ) effect imaging.The SZE data are extracted from the first public all-sky data release of Planck and we use high-frequency radio data at 2.3 GHz to constrain the shock-front geometry. The SZE data provide evidence for a pressure discontinuity, consistent with the relic position, without requiring any additional prior on the shock-front location. The derived Mach number M = 2.9 (+0.8/-0.6) is consistent with X-ray and radio results. A high-pressure "filament" without any pressure discontinuity is disfavoured by X-ray measurements and a "sub-cluster" model based on the infalling group NGC 4839 can be ruled out considering the published mass estimates for this group. These results signify a first attempt towards directly measuring the pressure discontinuity for a radio relic and the first SZ-detected shock feature observed near the virial radius of a galaxy cluster.

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Magnification and Shear with the CFHTLenS Clusters

Presenter: Jes Ford
Authors: Jes Ford


         The magnification component of weak lensing provides complementary information to the more commonly measured shear distortion. We present recent measurements of halo masses from the stacked magnification and shear signals of >18,000 galaxy clusters in the 154 deg^2 Canada-France-Hawaii-Telescope Lensing Survey (CFHTLenS). We perform the first direct mass comparison between magnification and shear for such a large cluster sample. We find global agreement between the independent methods, but with systematic effects influencing particular cluster redshift ranges. We measure the mass-richness scaling relation of the CFHTLenS 3D-MF cluster sample, and search for evidence of its evolution with redshift.

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Treasures in radio surveys: search and discovery of double radio-relic systems

Presenter: Francesco de Gasperin
Authors: M. Bruggen, R. van Weeren, A. Bonafede, H. Intema


         Radio halos and radio relics are Mpc-scale radio sources in galaxy clusters. They trace the presence of non-thermal components (cosmic rays and magnetic field) in dynamically disturbed galaxy clusters.We performed a search in several radio surveys for radio halos and radio relics in all newly discovered SZ-selected Planck clusters and also all the previously known clusters. In this talk I report the first outcomes of this search which are the discovery of two new double radio relic systems, one of which hosts the most powerful radio relic system known so far, and the discovery of a rare radio phoenix in the moment of its formation. Results from several follow-ups in radio/optical/X-ray will be discussed.



Resolved SZ observations of two cool cores - are cold fronts what we think they are?

Presenter: Simona Giacintucci


         We present CARMA high-resolution SZ maps of two massive cool cores with well-modeled or faint central radio sources. While there is large-scale agreement with the X-ray, the peak of the SZ signal does not coincide with the X-ray peak, but instead sits at the tip of a sloshing cold front in both cluster. Theoretically, it is very difficult to generate small-scale pressure nonuniformities in a non-merging cluster; something significant must be missing from our understanding of the cool core physics.



Hypervelocity Globular: A beacon of merging clusters

Presenter: Oleg Gnedin
Authors: Oleg Gnedin


         A star cluster with an extraordinary blueshift of -1000 km/s was discovered last year by Caldwell et al. in a radial velocity survey of globular clusters in the Virgo Cluster. The origin of such a high negative velocity remains unknown. All exotic scenarios, including a close three-body encounter with binary black holes at the center of M87 galaxy, can be ruled out. I show instead that a more natural, and plausible, origin of this object is in the globular cluster system of the M86 group that is merging along the line of sight with the Virgo Cluster. Evidence for such a merger is supported by X-ray data. I will discuss further implications of extreme velocity outliers in highlighting cluster mergers.


Golovich - Poster

ZwCl 0008.8+5215: A spectroscopic and optical study with a dynamical analysis of a major cluster merger

Presenter: Nathan Golovich

         Galaxy clusters have proven to be a laboratory for the study of many interesting phenomena (particle acceleration, star formation, plasma physics, etc), but perhaps the most important aspect is the ability to test for a self-interaction nature of dark matter. However, to properly constrain a cross section, we must understand the dynamics of the merger. Radio relic selection has shown it can identify clean, bimodal systems, which enable us to provide reasonable constraints on the merger dynamics. The Merging Cluster Collaboration has set out to study a sample of such systems, and ZwCl 0008.8+5215 is a textbook example. Situated in the plane of the sky with a double relic, it hosts bimodal structure with near equal dynamical mass (4.5E14 and 5E14 solar masses respectively). A dynamical Monte Carlo analysis shows it to be near turnaround 1 Gyr after a merger event with a collisional velocity of 1800 km/s. We will present these details on the stage of our studies of other merging systems, present joint Subaru/HST weak lensing masses along with constraints on a dark matter-galaxy offset, and discuss future studies of this system.



Prospects for Current and Future Infrared Galaxy Cluster Surveys

Presenter: Anthony Gonzalez


         During the past decade, infrared galaxy surveys have provided anefficient means of identifying samples of galaxy cluster extending tohigh redshift and low mass, including many of the most distantclusters presently known. In this talk I will present the status ofcurrent surveys to detect clusters via their galaxy populations atinfrared wavelengths, emphasizing current programs to detecthigh-redshift galaxy clusters with data from Spitzer and WISE. I willalso discuss ways in which these programs provide a usefulcomplementarity to ICM-based searches for assessing potentialselection effects and biases in scaling relations, and considerprospects for next-generation infrared cluster surveys with upcomingmissions.


Griffin - Poster

New Limits on Gamma-Ray Emission from Galaxy Clusters

Presenter: Rhiannon Griffin
Authors: Rhiannon D. Griffin, Xinyu Dai, Chris S. Kochanek

         Galaxy clusters are predicted to produce gamma-ray emission, through cosmic ray interaction and/or dark matter annihilation, potentially detectable by the Fermi Large Area Telescope (Fermi-LAT). Specifically, cosmic ray interactions with the intra-cluster gas results in neutral pion decay, producing gamma-rays. As yet, this emission has not been detected using small samples of clusters. We present a new, independent stacking analysis of Fermi-LAT photon count maps using the 78 richest nearby clusters (z < 0.12) from the Two Micron All-Sky Survey (2MASS) cluster catalog. Our initial search yields non-detections of gamma-ray emission from galaxy clusters but we achieve the lowest upper limits on the photon flux to date. Scaling to recent cosmic ray acceleration and gamma-ray emission models, we find that cosmic rays represent a negligible contribution to the intra-cluster energy density and gas pressure. Furthermore, either merger shocks must have lower Mach numbers than inferred from radio emission, so < 2 - 4, or significantly less than 50% of the baryon mass has been processed through such shocks, and thus, the majority of baryons should be assembled through minor mergers or through cold accretion.



Probing AGN feedback and the ICM viscosity with the shape of X-ray cavities in galaxy clusters

Presenter: Fulai Guo
Authors: Fulai Guo


         Detections of numerous X-ray cavities in galaxy clusters provide the most compelling evidence for the importance of mechanical AGN feedback. Here we use a large suite of hydrodynamic simulations to argue for the first time for the physical importance of the cavity shape. We show that it could be used to probe the properties of AGN jets and the viscosity level in the intracluster medium. Bottom-broad cavities elongated along the angular direction are created by very light internally-subsonic jets, while top-broad cavities elongated along the jet direction are produced by heavier jets with internally supersonic velocities. The long-term cavity evolution is significantly affected by the level of ICM viscosity, which helps suppress interface instabilities. We encourage observers to study the shapes of X-ray cavities, which may shed new insights on the properties of AGN jets and the ICM viscosity level. (arXiv:1408.5018).

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Radio jet interactions with the hot ICM and the evolution of radio feedback over cosmic time

Presenter: Julie Hlavacek-Larrondo
Authors: Julie Hlavacek-Larrondo


         The last few decades have shown us that radio jets originating from supermassive black holes can interact strongly with their surrounding medium. During this talk, I will review the current status of this field, known as AGN feedback, while focusing on the most massive black holes in the Universe, those that lie at the centers of galaxy clusters. I will review the physics behind these interactions and then focus on the evolution of radio feedback over cosmic time.



Accurate cluster masses from weak lensing

Presenter: Henk Hoekstra


         Masses of clusters of galaxies from weak gravitational lensing analyses of ever larger samples are increasingly used as the reference to which baryonic scaling relations are compared. These provide a critical connection between cluster surveys and the theory of structure formation, necessary to infer cosmological parameters. I will discuss the challenges in achieving percent precision by highlighting recent progress in the (re-)analysis of the Canadian Cluster Comparison Project.


Hofmann - Poster

Galaxy cluster substructure study with Chandra

Presenter: Florian Hofmann
Authors: F. Hofmann, J.S. Sanders, K. Nandra, N. Clerc

         In high resolution X-ray observations of the hot plasma in clusters of galaxies significant substructure caused by AGN feedback, mergers and turbulence can be detected. Since the start of the Chandra mission in 1999 many clusters have been observed in great depth and at high resolution.Using archival data taken with the Chandra ACIS instrument our aim was to study the substructures in thermodynamic properties of the X-ray emission from clusters of galaxies and to apply this to better understand the thermodynamic and dynamic state of the intra cluster medium.We analyzed deep observations for a sample of 33 clusters with more than 100 ks of Chandra exposure each at distances between redshift 0.025 and 0.45. The combined exposure of the sample is 8 Ms. Fitting emission models to different regions of the extended X-ray emission we searched for substructures in density, temperature, pressure, and entropy of the hot plasma.For individual clusters we mapped thermodynamic properties of the intra cluster medium and investigated deviations from radial symmetry around the cluster centers. The relation of asymmetries in different parameters can be used as an indicator for turbulence.This study provides detailed asymmetry measurements for a large sample of clusters which will be used to study turbulence in the systems and make predictions for future X-ray missions like eROSITA.



On the Trail of the Most Massive Clusters in the Universe

Presenter: John P. Hughes
Authors: John P. Hughes, Felipe Menanteau, Felipe Barrientos, Leopoldo Infante


         In March 2013, the Planck Collaboration released the first large sample of Sunyaev Zel'dovich (SZ) Effect-selected galaxy clusters from a survey covering essentially the entire sky. The Planck SZ (PSZ) catalog contains 861 confirmed clusters, as well as another 366 unconfirmed candidates. The confirmed sample includes the most massive known clusters and extends up to redshifts as high as z~0.9. The Planck confirmation process favored candidates with some signal in ancillary data sets, which resulted in the preferential confirmation of low-redshift systems. The unconfirmed sample, therefore, is likely biased toward massive clusters at redshifts z>0.5. Over the past year we have been actively pursuing the identification of unconfirmed Planck cluster candidates using optical and near-infrared imaging observations on 4-m class telescopes (the Mayall telescope on Kitt Peak and the SOAR telescope on Cerro Pachon). In this talk we report on our study aimed at unveiling all the most massive galaxy clusters in the Universe.This work is supported by NASA ADAP grant NNX14AF73G.

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Observing Galaxy Clusters with Next Generation Radio Telescopes

Presenter: Johnston-Hollitt


         The rapid expansion of radio telescopes in the last decade has given rise to an unprecedented opportunity to advance a number of topics related to galaxy clusters and more importantly connect these advances with emerging facilities at other wavelength regimes. I will present an overview of my recent work examining galaxy clusters using current radio telescopes and discuss opportunities to exploit the science of clusters further with the MWA, ASKAP and ultimately the SKA.


Jones, C

Characterizing and Comparing the Planck SZ Selected Clusters with X-ray and Optically selected Cluster Samples

Presenter: C. Jones
Authors: C. Jones, F. Andrade-Santos, W. Forman, R. van Weeran, S.S. Murray for the Chandra-Planck XVP Consortium


         The Planck mission provided a representative sample of clusters of galaxies over the entire sky. With completed Chandra observations of 165 Planck ESZ clusters at z<0.35, we are characterizing each cluster in terms of its X-ray luminosity, gas temperature, gas mass, entropy, central cooling time, presence of active AGN, radio emission, gas cavities and cluster morphology. We also determine the total cluster mass, primarily through scaling relations. We measure the fraction of Planck-detected clusters with different cluster morphological types, ranging from cool core relaxed systems to disturbed major mergers. We compare the fractions of clusters with relaxed and disturbed morphologies in X-ray and optically selected samples with the Planck ESZ sample. We also compare the fractions of clusters with cool cores and gas cavities in the Planck and X-ray selected samples. Although we do not find significant differences between the fractions of clusters with disturbed morphologies due to merging in different samples, we do find a smaller fraction of cool core clusters in the Planck sample than in X-ray selected cluster samples.


Jones, C

The X-ray Surveyor/SMART-X Mission

Presenter: C. Jones for the X-ray Surveyor/SMART-X Team

         For 15 years, Chandra has explored the high energy Universe with ~half-arcsecond resolution. Chandra studies have deepened our understanding of clusters, galaxies, AGN, supernova remnants, and stars, encompassing most areas of astrophysics. While there has been very significant progress, it is also clear that many Chandra observations are photon limited. A future high angular resolution mission, with significantly more collecting area along with the next generation of science instruments is needed to study the formation and growth of black holes at high redshifts, the evolution of galaxies, the growth of groups and clusters and the details of AGN feedback on cluster, group and galaxy scales. The X-ray Surveyor/SMART-X approach will demonstrate a way to build sub-arcsecond imaging large-area, low-mass, reasonable-cost X-ray mirrors. We will summarize the cluster related science that such a mission could address and the technical progress that has been achieved.


Jones, T

Acceleration of CRs in galaxy clusters and their non-thermal emissions

Presenter: Tom Jones
Authors: T. W. Jones


         I will review the current status of and some recent contributions to our understanding of cosmic ray acceleration in galaxy clusters by way of shocks and turbulence, as well as the connections between those processes and diffuse non-thermal cluster emissions, including radio halos and peripheral radio relics. Work on this topic by the author is supported by the NSF and the University of Minnesota Supercomputing Institute.

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Halo mass definitions and scaling relations

Presenter: Andrey Kravtsov
Authors: Andrey Kravtsov, Benedikt Diemer, Surhud More


         I will review discuss different choices of how halo mass can be defined and effect of these choices on the halo scaling relations. I will present a new physical mass definition, which takes into account all of the mass physically accreted by a halo by a given epoch.



Progress in ICM Plasma Physics

Presenter: Matthew Kunz

         At the last Snowcluster two years ago, I gave a talk with an obnoxious title ("The ICM from a plasma physicist's perspective, and why you should care"). In it, I sought to convince the audience that, not only is the detailed plasma physics of the ICM worth knowing, but also that we are on the verge of figuring it out. Despite the as-yet-unfulfilled promise that "very soon, saying `it's complicated' will no longer be an option", substantial progress has in fact been made in understanding how microscale plasma processes affect the macroscale transport properties of weakly collisional, magnetized plasmas (such as the ICM). Much of this progress is due to the increasing use of kinetic particle-in-cell codes by the astrophysics community; I will present some of my contributions using one such code (Pegasus). Recent efforts to construct a theoretical framework for low-frequency electromagnetic (gyro-)kinetic turbulence in plasmas like the solar wind and the ICM - with implications for the differential heating of ions and electrons - will also be highlighted.

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Lau - Poster

Measuring Gas Motions and Hydrostatic Mass Bias with ASTRO-H

Presenter: Erwin Lau
Authors: Erwin Lau, Daisuke Nagai, Naomi Ota


         Cosmological constraints from X-ray and Sunyaev-Zel'dovich cluster surveys are limited by uncertainties in cluster mass estimates based on the assumption of the hydrostatic equilibrium (HSE). The HSE mass bias originates primarily from the non-thermal pressure support provided by gas motions in galaxy clusters generated by mergers and accretion, which are not accounted for in the current cluster-based cosmological measurements. The ASTRO-H X-ray mission, equipped with high spectral resolution calorimeter, will be able to directly measure gas motions in galaxy clusters for the first time and the correction of the HSE mass bias arising from gas motions. Since gas motions in galaxy clusters are complex in nature, measurements of gas motions and HSE mass bias with ASTRO-H are likely to be subjected to systematic biases and uncertainties. Physically meaningful measurements of cluster gas motions and HSE mass bias require detailed studies of mock simulations of galaxy clusters that realistically captures physical processes that generates gas motions in clusters, such as mergers and accretion. In this work, we analyze mock ASTRO-H simulations of galaxy clusters extracted from cosmological hydrodynamic simulations and assess how well ASTRO-H will be able to measure line-of-sight bulk and turbulent motions of the ICM. We then demonstrate how well HSE mass bias can be recovered with ASTRO-H gas motion measurements. Our work opens up the possibility of recovering accurate cluster mass with ASTRO-H, and future X-ray mission with high resolution X-ray spectroscopy such as Athena+ and SMART-X.


Le Brun

Scatter and evolution of the hot gas properties of a realistic population of simulated groups and clusters

Presenter: Amandine Le Brun
Authors: Amandine Le Brun, Ian McCarthy, Joop Schaye, Trevor Ponman


         We present an investigation of the scatter and evolution of the hot gas properties (as probed by X-ray and Sunyaev-Zel’dovich observations) of galaxy groups and clusters as a function of the important non-gravitational physics of galaxy formation using the cosmo-OWLS suite of cosmological hydrodynamical simulations. We have previously shown (Le Brun et al. 2014, McCarthy et al. 2014) that a subset of our simulations which includes efficient AGN feedback reproduces a very wide range of properties of the local group/cluster population. We find that the predictions of the self-similar model break down when efficient feedback is included, for both mass slope and evolution, that the log-normal scatter varies only mildly with mass and non-gravitational physics but displays a relatively strong redshift dependence (it tends to decrease with increasing redshift), and that X-ray temperature is the best overall mass proxy while X-ray luminosity is the poorest.



Cooling, AGN Feedback and Star Formation in Cool-Core Galaxy Clusters

Presenter: Yuan Li
Authors: Yuan Li, Greg L. Bryan, Mateusz Ruszkowski, Mark Voit, Brian O’Shea

         The feedback from active galactic nuclei (AGNs) is widely considered to be the major heating source in cool-core galaxy clusters, preventing a classical cooling flow where the intra-cluster medium (ICM) cools at hundreds to a thousand solar masses per year. Numerical simulations with AGN feedback have successfully suppressed radiative cooling, but generally fail to reproduce the right amount of cold gas and the expected cyclical AGN activities. We perform adaptive mesh simulations using Enzo including both momentum-driven AGN feedback and star formation to study the interplay between cooling, AGN heating and star formation over 6.5 Gyr in an isolated cool-core cluster. Cold clumps first cool out of the ICM due to the non-linear perturbation driven by the AGN jets. These cold clumps feed both star formation and the supermassive black hole (SMBH), triggering an AGN outburst which increases the entropy of the ICM and reduces its cooling rate. Within 1-2 Gyr, star formation completely consumes the cold gas, leading to a brief shutoff of the AGN. The ICM quickly cools and develops multiphase gas again, followed by another cycle of star formation/AGN outburst. Within 6.5 Gyr, we observe three such cycles. The star formation rate (SFR) is correlated with the total amount of cold gas in the system. The average star formation rate is 40 solar masses per year. The black hole accretion rate shows a large scatter, but the average correlates well with the star formation rate and is roughly one order of magnitude lower.


Lumb - Poster

XMM-Newton recalibration – prospects for fixing the Planck mass function problem

Presenter: D Lumb
Authors: D Lumb

         A solution for the tension in cosmological constraints reported by the Planck team between their SZ-selected cluster counts and Cosmic Microwave Background (CMB) temperature anisotropies remains elusive. Nearly fifteen years after their launch, important effective area mis-calibrations between Chandra and XMM-Newton and between XMM instruments also remain unresolved. I report recent investigations on the XMM-Newton legacy datasets to understand effects of stray light sieve misalignment, for on-axis effective area, off-axis vignetting and out of field stray light. All have important impacts for cluster data analysis with XMM-Newton.

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Planck all-sky y-map and interpretation

Presenter: Macias-Perez
Authors: J.F. Macias-Perez on behalf of the Planck collaboration


         We have constructed all-sky Compton parameters maps (y-maps) of the thermal Sunyaev-Zeldovich (tSZ) effect by applying specifically tailoredcomponent separation algorithms to the 30 to 857 GHz frequency channel maps from the Planck satellite survey. These reconstructed y-maps are delivered as part of the Planck 2015 release. The y-maps are characterized in terms of noise properties and residual foreground contamination, mainly thermal dust emission at large angular scales and CIB and extragalactic point sources at small angular scales. Specific masks are defined to minimize foreground residuals and systematics. Using these masks, we compute the y-map angular power spectrum and higher order statistics.From these we conclude that the y-map is dominated by tSZ signal in the multipole range, 20 < ` < 600. We compare the measured tSZ powerspectrum and higher order statistics to various physically motivated models and discuss the implications of our results in terms of cluster physics and cosmology.



Mass Profiles of Relaxed and Non-Relaxed Clusters of Galaxies

Presenter: Andy Mahdavi
Authors: A. Mahdavi, H. Hoekstra, A. Babul, J. P. Henry


         We present a multiwavelength, X-ray + weak lensing measurement of the mass profiles for 50 rich systems. Using the Joint Analysis of Cluster Observations codebase, we simultaneously model the baryonic and nonbaryonic matter profiles in these systems, deriving joint constraints on the gas entropy, pressure, metallicity, and dark matter distributions. Simultaneous analysis (and updated cross-calibration) of Chandra and XMM-Newton data where both are available allows us to constrain these profiles over nearly two decades in radius. We find clusters with low BCG-to-X-ray center offsets form a remarkably regular sample, with NFW dark matter profiles and gas fraction values that are consistent with the cosmological value. Clusters with low central gas entropy exhibit a similar trend, and do so with an intrinsic scatter that is consistent with zero. Non-relaxed clusters, on the other hand---those with offset BCGs and high central entropies---exhibit significant scatter and have mass profiles less consistent with the NFW value (the most likely effect likely being due to strong violations of spherical symmetry).



Cosmological Constraints from Galaxy Clusters

Presenter: Adam Mantz


         Galaxy clusters provide multiple, independent probes of cosmology, and are sensitive to both the expansion of the Universe and the growth of cosmic structure. I will review recent developments in this field, namely the availability of accurate mass estimates from weak lensing and of large-area surveys utilizing the Sunyaev-Zel'dovich effect, and present the results on dark matter, dark energy, gravity and neutrino masses.



A possible detection of inverse Compton signal from a radio relic, and new constraints on large-scale heat conduction in clusters

Presenter: Maxim Markevitch
Authors: Maxim Markevitch


         Title: I will show a possible first detection of inverse Compton emission from one of the famous radio relics with Chandra (the evidence is tantalizing, but marginally significant). I will also show how the recently demonstrated low scatter in the observed gas mass fraction within r_2500 can be used to place an upper limit on the large-scale heat conduction in galaxy clusters.



High Resolution SZE Imaging of Galaxy Clusters with MUSTANG and MUSTANG-2

Presenter: Brian Mason
Authors: Brian Mason, Simon Dicker, Charles Romero, Tony Mroczkowski, Alex Young, Sara Stanchfield, Erik Reese, Justus Brevik, Jack Sayers, Nicole Czakon, Craig Sarazin, Mark Devlin


         For the past several years our collaboration has been carrying out acampaign of high angular resolution (10") imaging of the SZE in galaxyclusters with the MUSTANG bolometer array on the GBT. Thesemeasurements, focusing mainly on the CLASH sample, provide a view of the ICM state and dynamics which complements that provided by other common probes (x-ray imaging and spectroscopy and lower resolution SZE imaging). I will present results from this survey to date, including ICM pressure profile measurements obtained by combining MUSTANG and BOLOCAM data, and discuss work that is underway. Finally I will describe results from, and plans for, the newly commissionedMUSTANG-2 receiver.



Spectral-imaging of a nearby sample of galaxy clusters observed with Planck

Presenter: Pasquale Mazzotta
Authors: Pasquale Mazzotta


         The Planck early and intermediate results have proven the strength of the Sunyayev-Zeldovich (SZ) effect in detecting the hot gas emission out to the galaxy cluster peripheries, opening a new window towards the complex baryonic physics at play in these regions where the dark matter halo form and continuously accrete substructures. To search for gas density anisotropies related to the filamentary accretion of materials, or pressure discontinuities induced by the propagation of shock fronts, we developed a spectral-imaging algorithm dedicated to the SZ observations of nearby galaxy clusters with Planck. To optimize the inescapable compromise between angular resolution and precision of the SZ flux measurements, this algorithm performs a multiscale analysis of the SZ maps together with the additional components that appears as mixed up with the SZ signal. More precisely, the CMB anisotropies are analyzed through a wavelet transform, while the Galactic foregrounds and SZ maps are analyzed via a curvelet transform that best preserves their anisotropic details. Using mock Planck observations, we tested the performance of the algorithm data sets that mimic the signal-to-noise ratio of both the nominal 15.5 months and the extended 30 months Planck mission.We, then, applied this new technique to map a sample of ~ 10 nearby systems (including Coma, A3667, A2142) derived from the public nominal 15.5 months Planck data set. We will discuss these new results comparing them with a complementary component separation algorithm based on Internal Linear Combination Method and with the respective X-ray images.



Probing Galaxy Cluster Evolution with Combined X-ray+SZ Surveys

Presenter: Michael McDonald
Authors: Michael McDonald


         We present the latest results from combined surveys with the Chandra X-ray Observatory and the South Pole Telescope. Playing to the strengths of both facilities, we have obtained deep, high-angular resolution X-ray imaging spectroscopy for a sample of >90 clusters in a mass-limited sample spanning 0.3 < z < 1.2. These data reveal, for the first time, the evolution of the average gas density, temperature, entropy, and pressure profiles out to z > 1. Additional constraints on the evolution of AGN feedback, cluster interactions, metallicity, and baryon fractions will be presented. Finally, ongoing efforts to extend this survey to 1.2 < z < 1.7, and future efforts to expand the number of detected clusters by orders of magnitude, will be discussed.



AGN Heating and Scaling Relations

Presenter: McNamara
Authors: No abstract


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Meece - Poster

Self Regulating AGN Feedback in Cool-Core Clusters

Presenter: Greg Meece
Authors: Gregory Meece, Brian O'Shea, Mark Voit


         Many cool-core galaxy clusters exhibit central cooling times much shorter than the age of the cluster, yet do not contain the large amounts of cold gas or show evidence of the star formation which would be expected if the ICM were indeed cooling. We present a simulations of a self regulating feedback cycle in which AGN are responsible for balancing radiative cooling and maintaining thermal balance. In this model, gas condensing out of the cooling ICM is accreted onto the SMBH, triggering AGN feedback that heats the ICM and restores thermal balance. Using state of the art hydrodynamics simulations in both idealized and cosmological settings, we study whether this mechanism is capable of robustly balancing cooling and reproducing the observed properties of galaxy clusters.




Presenter: Massimo Meneghetti
Authors: Massimo Meneghetti and the MUSIC and CLASH teams


         I will discuss how the recent measurements of the mass profiles of the CLASH clusters compare with theoretical expectations from the MUSIC simulation suite. In particular, I will focus on the concentration-mass relation and on the distributions of Einstein radii. I will discuss the impact of several selection effects on the derivation of the theoretical c-M relation as well as the biases induced by projection effects.



The concentration-mass relation from CLASH

Presenter: Julian Merten
Authors: The CLASH team


         Clusters of galaxies, as the most massive gravitationally bound structures in the observable Universe, are sensitive probes of structure formation scenarios. The Cluster Lensing and Supernova Survey with Hubble (CLASH) maps the matter distribution in 25 rich galaxy clusters using gravitational lensing measurements derived from a combination of multi-band HST and Subaru imaging. This combination of data allows for a reliable multi-scale reconstruction of the dark matter density distribution over three decades in radius - from the central 50 kpc out to beyond the cluster virial radius. We present constraints the cluster mass-concentration (M-c) relation for an x-ray selected subset of the full CLASH sample (20 of the 25 clusters). The concentration parameter is defined as the spatial extend of the full cluster halo divided by the spatial extent of the halo core and can be analyzed as a function of total halo mass and redshift. Former studies have suggested a strong tension in the observed M-c relation when compared to expectations derived from numerical simulations. We will present our current constraints on the M-c relation along with comparisons to the latest theoretical expectations.



Simulations of turbulence in galaxy clusters

Presenter: Francesco Miniati


         I will present recent work on simulations of resolved turbulence in galaxy clusters. Turbulence holds the key to understanding several important aspect of ICM physics. In this talk I will use the simulation results to discuss in quantitative terms the role of turbulence and, to a lesser extent, the ICM microphysics on the origin of magnetic field and diffuse radio emission.


Mo - Poster

Weak Lensing Analysis of IDCS J1426.5+3508: A Massive Galaxy Cluster at z=1.75

Presenter: Wenli Mo
Authors: W. Mo, A. Gonzalez, M. J. Jee, R. Massey, J. Rhodes, M. Brodwin, P. Eisenhardt, D. Marrone, S. A. Stanford, G. R. Zeimann

         We present a weak lensing study of the galaxy cluster IDCS J1426.5+3508 which, at z=1.75, is the highest redshift strong lensing cluster known and the most distant cluster for which a weak lensing analysis has been undertaken. Using F160W, F814W, and F606W observations with the Hubble Space Telescope, we detect a tangential shear signal out to a radius of ~1 Mpc. Fitting the shear to a Navarro-Frenk-White mass profile, we derive a mass that agrees with previous masses calculated from the Sunyaev-Zel'dovich (SZ) effect, X-ray, and strong lensing observations. The combination of weak lensing and SZ measurements are consistent with the scaling relation observed at low redshift, indicative of minimal evolution in this relation. The cluster concentration is not well-constrained by the weak lensing data alone, but we derive a concentration using the SZ mass measurements as a prior. It has previously been noted that an exceptionally high concentration might afford a means to explain the unexpected existence of a giant arc in this cluster. Our data argue that the concentration is not unusually large, indicating that an alternate solution is required.



Cluster outskirts: light at the end of the tunnel?

Presenter: Silvano Molendi
Authors: Silvano Molendi


         After briefly reviewing published results on cluster outskirts I shallfocus on recent work. I will present new ROSAT + Planck profiles for A2319, where we have a firm detection of cluster emission out to at least 3xr500 for Planck and 2xr500 for ROSAT. I will also present results from the analysis of SWIFT XRT data of A2244 out to r200. Swift, like Suzaku, benefits from the low background typical of equatorial low earth orbit. An important point in our analysis is that, at variance with what is done for Suzaku data, we perform a full modeling of the background, including the instrumental component. Finally I will show, through an example, how the application of a similar approach, i.e. instrumental background modeling rather than substracting, impacts on Suzaku measures of thermodynamic quantities.



The galaxy cluster outskirts via stacking

Presenter: Andrea Morandi
Authors: Andrea Morandi, Ming Sun, William Forman, Christine Jones


         Exploring the virialization region of galaxy clusters has recently raised the attention of the scientific community, offering a direct view of structure formation. In this talk, I will present recent results on the physical properties of the intracluster medium in the outer volumes of a sample of 320 clusters (0.056 < z < 1.24, kT > 3 keV) in the Chandra archive, with a total integration time of ~20 Ms. We stacked the emission measure profiles of the clusters to detect a signal out to R_{100}. We then measured the average emission measure, gas density and gas fraction, which scale according to the self-similar model of cluster formation. We observe a steepening of the density profiles beyond R_{500} with slope beta ~ 0.68 at R_{500} and beta ~ 1 at R_{200} and beyond. By tracking the direction of the cosmic filaments where the clusters are embedded, we report that galaxy clusters deviate from spherical symmetry. We also did not find evolution of the gas density with redshift, confirming the self-similar evolution of the gas density. The value of the baryon fraction reaches the cosmic value at R_{200}: however, systematics due to non-thermal pressure support and clumpiness might enhance the measured gas fraction, leading to an actual deficit of the baryon budget with respect to the primordial value. We finally used, for the first time, the outskirts of clusters to constrain the cosmological parameters. We exploited the high-level of similarity of emission measure profiles in the hot cluster population as cosmological distance proxy, the correct cosmology being the one for which the various profiles at different redshifts coincide. This cosmological test has been compared to the standard gas fraction test to infer the impact of systematics. The present large dataset allows us to put a tight constraint on Omega_0=0.29+/-0.018 and on the density of the dark energy w=-0.95+/-0.06 at 68.3% confidence level with an unprecedented accuracy, comparable and complementary to other methods (e.g SN, CMB). This novel method, the stacking the X-ray signal of cluster outskirts, has the capacity to provide a generational leap forward in our understanding of cluster physics and formation, and the use of clusters as cosmological probes.



High Resolution SZ Effect Imaging with the Goddard-IRAM Superconducting 2-Millimeter Observer

Presenter: Tony Mroczkowski
Authors: Tracy Clarke, Harald Ebeling, Attila Kovacs, Johannes Staguhn, Alex Young, and many others from the ACT, eMACS, and GISMO Collaborations


         The Goddard-IRAM Superconducting 2-Millimeter Observer (GISMO) is a 2-mm (150 GHz) 8x16 element bolometer array that, until recently, operated from the IRAM 30-meter Telescope, providing a resolution of 16.5 arcseconds. In short 2-4 hour observations, it was able image the surface brightness decrement due to the Sunyaev-Zel'dovich effect from high-redshift massive clusters. Newly-built, the dual-band GISMO-2 instrument is ready to be fielded and will double its 2-mm pixel count, providing 256 detectors with lower instrument noise than the original GISMO. Additionally, GISMO-2 will simultaneously image the SZ increment and mm-wave sources with 1280 detectors operating at 1.2 mm (250 GHz). I will present GISMO SZ imaging of a couple samples of massive high-redshift clusters. The first sample includes clusters discovered in the Extended MAssive Cluster Survey (eMACS), culled from the ROSAT All-Sky Faint Source Catalogue and including some of the most X-ray luminous clusters at z>0.5. The second sample includes a number of merging clusters discovered by the 6-meter Atacama Cosmology Telescope (ACT) through their 1.4' resolution SZ survey of the SDSS Stripe 82 field. The GISMO observations indicate that ground-based, high-resolution SZ follow-up can complement more costly X-ray follow-up, providing confirmation of the hot ICM and enabling morphological studies.

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Gas Accretion and Non-equilibrium Phenomena in the Outskirts of Galaxy Clusters

Presenter: Daisuke Nagai
Authors: Daisuke Nagai


         In recent years, the outskirts of galaxy clusters have emerged as one of the new frontiers for studying the physics of clusters and intergalactic medium. In this talk, I will discuss the physics of galaxy clusters outskirts, focusing on roles of cosmic gas accretion and non-equilibrium phenomena, such as gas inhomogeneities, turbulence, and electron-proton equilibration process operating in the cluster outskirts. I will discuss implications for interpreting deep Chandra X-ray observations of A133.


Nelson - Poster

Effect of Mergers on Cluster Mass Estimates in Cosmological Simulations

Presenter: Kaylea Nelson

         Cosmological constraints from X-ray and microwave observations of galaxy clusters are subjected to systematic uncertainties. Non-thermal pressure support due to internal gas motions in galaxy clusters is one of the major sources of astrophysical uncertainties, which result in large bias and scatter in cluster mass estimates. In this work, we analyze a sample of massive galaxy clusters from the Omega500 high-resolution hydrodynamical cosmological simulation to examine the effects of dynamical state on non-thermal pressure. We use detailed merger trees that trace cluster mass assembly to examine the effects of mergers on the hydrostatic mass bias, the Y-M scaling relation and the evolution of non-thermal pressure. We find that during major mergers about a third of the total pressure support in the system is in non-thermal pressure from random gas motions, which leads to a ~30% bias in the hydrostatic mass estimate. Even after clusters relax, we find a residual 10% bias due to the residual non-thermal pressure sustained by continuous gas accretion and minor mergers in cluster outskirts. However, when the non-thermal pressure support is accounted for in the mass estimates of relaxed clusters, we are able to recover the true mass to within a few percent. Similarly, the evolution of the non-thermal pressure contributes significantly to the bias, scatter and evolution in the Y-M scaling relation. After mergers, the kinetic energy is slowly converted into thermal energy through dissipation of gas motions, which causes the thermal SZE signal to increase steadily with time. However, we show that when the missing non-thermal energy is accounted for, the resulting relation exhibits little to no redshift evolution and the scatter around the scaling relation is reduced. We also use merger trees to characterize the role of minor mergers and smooth accretion in sourcing low levels of sustained non-thermal pressure. We present a universal, redshift-independent fitting formula for estimating the fractional non-thermal pressure support in clusters. We will discuss implications of our results for the recent tension in the Planck CMB and galaxy cluster cosmological constraints.



The Dark Matter Distribution at the Centers of Groups and Clusters

Presenter: Andrew Newman
Authors: Andrew Newman, Richard Ellis, Tommaso Treu, David Sand


         Observations of the shape of the dark matter density profile provide a fundamental test of models of dark matter and galaxy formation. These measurements become challenging close to centers of halos where the contribution of baryons must be disentangled. I will show how combining multiple types of observations, including stellar dynamics, X-ray emission, and strong and weak lensing, allows the density profile in massive clusters to be constrained on scales of ~3 kpc - 3 Mpc. Surprisingly, we find that the total mass distribution in cluster cores follows the canonical CDM profile better than the dark matter alone, which has a shallower density profile on the scale of the central elliptical galaxy. I will discuss these results in light of recent numerical simulations, as well as a new observational program to extend our methods to lower mass halos.



The return of the merging galaxy subclusters of El Gordo

Presenter: Karen Y. Ng
Authors: Karen Y. Ng, William A. Dawson, D. Wittman, M. James Jee, John P. Hughes, Felipe Menanteau, Cristóbal Sifón


         Merging galaxy clusters with radio relics provide rare insights to the merger dynamicsas the relics are created by the violent merger process. We demonstrate one of the firstuses of the properties of the radio relic to reduce the uncertainties of the dynamicalvariables and determine the 3D configuration of a merging cluster, ACT-CL J0102-4915, nicknamed El Gordo. We employ a Monte Carlo simulation to investigate the three-dimensional (3D) configuration and dynamics of El Gordo. Using the polarization fraction of the radio relic, we constrain the estimate of the angle between the plane of the sky and the merger axis to be = 21+9 /-11 degree. We find the relative 3D merger speed of El Gordo to be 2400 +400/-200 km / s at pericenter. The two possible estimates of the time-since-pericenter are 0.46 +0.09 / -0.16 Gyr and 0.91 +0.22 / -0.39 Gyr for the outgoing and returning scenario respectively. From the double relic observation and the X-ray observation of a comet-like gas morphology induced by motion of the cool core, it is widely believed that El Gordo is observed shortly after the first core-passage of the subclusters. However, our estimates of the time-since-pericenter would imply the two subclusters are more likely to be moving towards, rather than away, from each other, post apocenter, under certain assumptions about the merger and shock wave speeds.I will also compare and contrast the merger scenario of El Gordo with that of the Bullet Cluster, and show that this late-stage merging scenario explains why the southeast dark matter lensing peak of El Gordo is closer to the merger center than the southeast cool core.(http://arxiv.org/abs/1412.1826)


Nugent - Poster

Hot Halo Emission Detected at Outskirts of Two Poor Galaxy Groups Using Suzaku

Presenter: Jenna Nugent
Authors: Jenna Nugent, Xinyu Dai, Ming Sun

         We present Suzaku off-center observations of two poor galaxy groups, NGC 3402 and NGC 5129, with temperatures below 1 keV. Through spectral decomposition, we measure their surface brightnesses and temperatures out to 420 and 300 times the critical density of the universe for NGC 3402 and NGC 5129, respectively. These quantities are consistent with extrapolations from existing inner measurements of the two groups. With the refined X-ray luminosities, the two groups prefer Lx-T relations without a break in the group regime. Furthermore, we measure the electron number densities and hydrostatic masses at these radii. We find that the electron number density profiles require three ß-model components, including one with a flat slope, for both groups. Adding the gas mass measured from the X-ray data and stellar mass from group galaxy members, we measure baryon fractions of fb = 0.120 and 0.116 for NGC 3402 and NGC 5129, respectively. Combining other poor groups with well measured X-ray emission to the outskirts, we find an average baryon fraction of fb,ave = 0.111 for X-ray bright groups with temperatures between 0.8-1.3 keV, extending existing constraints to lower mass systems.



Radio Jet Interaction with the Hot and Cold Intracluster Medium

Presenter: Paul Nulsen


         The physical processes underlying radio mode AGN feedback remain the subject of debate, despite the well-established empirical case for feedback. This presentation will focus on how jets from the central AGN can inhibit cooling of the hot intracluster medium in cool-core clusters. Viable jet heating mechanisms will be discussed. The existence of the recently proposed "precipitation" threshold would largely rule out locally stable heating models. Evidence for outflows of hot and cold gas induced by the jets will be reviewed briefly. Mechanisms by which jets can set molecular gas in motion will be outlined and the role cold gas flows in the AGN feedback cycle discussed.

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Confronting Theory with X-ray Observations of Cluster Merger Shocks

Presenter: Georgiana Ogrean
Authors: G. Ogrean, R. van Weeren, H. Rottgering, M. Bruggen, A. Simionescu, M. Hoeft, J. Croston


         Structure growth in the universe is one of the fundamental astrophysical problems. Merging galaxy clusters are the perfect laboratories to study the effects of structure growth on the thermal and the non-thermal particle populations in the ICM. Cluster mergers trigger weak shocks in the ICM, which can accelerate particles to relativistic energies to form radio relics. At the moment, the shock acceleration model is the one most commonly used to explain the shock-relic connection. However, recent observational results challenge our understanding of shocks and radio relics, and require further theoretical investigation. I will review some of the most puzzling results obtained from joint X-ray and radio observations of clusters hosting radio relics -- including the Sausage cluster, the Toothbrush cluster, ZwCl 2341.1+0000, and Abell 3411 -- and discuss how these results fit into our current understanding of particle acceleration.


Peng Oh

The Origin of Seed Electrons in Radio Halos and Relics

Presenter: S. Peng Oh
Authors: S. Peng Oh (presenting), Anders Pinzke, Christoph Pfrommer


         About 1/3 of X-ray-luminous clusters show smooth, unpolarized radio emission on Mpc scales, known as giant radio halos. One promising model for radio halos is Fermi II acceleration of seed relativistic electrons by turbulence of the intracluster medium (ICM); Coulomb losses prohibit acceleration from the thermal pool. However, the origin of seed electrons has never been fully explored. Here, we integrate the Fokker-Planck equation of the cosmic ray (CR) electron and proton distributions in a cosmological simulations of cluster formation to probe the origin of seed electrons. We find that significant modifications to standard scenarios are required to produce the appropriate population of seed electrons. We also consider the possibility that radio relics arise from shock acceleration of fossil relativistic electrons.



Cluster science expected from ASTRO-H

Presenter: Takaya Ohashi
Authors: T. Ohashi


         ASTRO-H is planned for launch in early 2016, and the first observations from X-ray microcalorimeters with an energy resolution better than 7 eV will be carried out. Combined studies with microcalorimeters and hard X-ray telescopes will show dynamical and non-thermal features of clusters clearly for the first time. We will describe the status, operational plan of the mission and expected sciences on clusters from ASTRO-H.

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Discovery of O VII line emission from cooling gas in elliptical galaxies

Presenter: Ciro Pinto
Authors: Ciro Pinto


         In the cores of ellipticals, clusters, and groups of galaxies, the gas has a cooling time shorter than 1 Gyr. It is possible to probe cooling flows through the detection of Fe XVII and O VII emission lines, but so far O VII has not been detected in any individual object. The Reflection Grating Spectrometers (RGS) aboard XMM-Newton are currently the only instruments able to detect O VII in extended objects such as elliptical galaxies and galaxy clusters. In this talk I will show a recent discovery of O VII resonance and forbidden emission lines in a small sample of elliptical galaxies observed with XMM-Newton. This detection reveals for the first time cooling of X-ray emitting gas below 0.4 keV. Both O VII and Fe XVII resonance lines are partly suppressed in these sources due to resonant scattering. Our results suggest that the O VII and Fe XVII emission lines should be produced by the same cooling gas with mass deposition rates of 0.2-2 Msun/yr in agreement with the simulations. These cooling flows are compatible with the upper-limits obtained for several clusters of galaxies, but they are more difficult to detect due to the larger distances of the clusters as compared to the ellipticals in our sample.




Presenter: Etienne Pointecouteau
Authors: E. Pointecouteau


         The Athena mission will implement The Hot and Energetic Universe science theme selected by ESA for its second large-class mission in the Cosmic Vision program, due for launch in 2028. This theme poses two key astrophysical questions: 1) How does ordinary matter assemble into the large-scale structures we see today? and 2) How do black holes grow and shape the Universe? I will present the Athena mission, its instrument in view of these scientific objectives focusing on the science of galaxy clusters.



Multi-wavelength scaling relations

Presenter: Gabriel Pratt
Authors: G.W. Pratt


         Scaling relations can give unique insights into cluster physics. However, it is becoming increasingly clear that observational biases, including selection effects, must be accounted for in order to gain the clearest insights. I will give an overview of our current understanding of multi wavelength scaling relations, emphasising recent progress and highlighting present and future problem areas.

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Probing the Outskirts of Strongly Merging Double Clusters

Presenter: Scott W. Randall
Authors: E. Bulbul, E. Miller, R. Paterno-Mahler, C. Jones, W. Forman, C. Sarazin, S. Murray, E. Blanton


         We present very early results from Suzaku observations of a small sample of strongly merging "double" clusters (early stage, roughly equal mass mergers close to the plane of the sky) out to their virial radii. Most of these systems are in fact triple clusters, with all subclusters lying roughly along the same line, suggesting the presence of large scale structure filaments. Previous studies of other (non-merging) systems have found entropy profiles that flatten at large radii, in contradiction with theoretical predictions, possibly due to the presence of unresolved cool gas clumps. This behavior shows some variation with azimuth, suggesting a connection with large scale structure and gas accretion. We use our observations of these double clusters out to their virial radii, both along and perpendicular to their putative large scale structure filaments, to look for correlations between the ICM properties and the surrounding large scale environment.



High-z simulated cluster

Presenter: Elena Rasia
Authors: Nhut Troung, Susana Planelles, Pasquale Mazzotta, Stefano Borgani


         Current and forthcoming missions will discover and observe several high redshift clusters. I will review some theoretical predictions of the structural and thermodynamical properties of the most massive objects found in our simulations at redshifts from 1 to 2.



Understanding the ICM physics of low mass galaxy groups and distant galaxy clusters for and with eROSITA

Presenter: Thomas Reiprich
Authors: Thomas Reiprich


         Massive galaxy cluster scaling relations at low redshift are fairly well established today, the major uncertainty still remaining in the total mass determination. The rewarding frontiers ahead of us include the robust and precise extension of these scaling relations to lower masses (galaxy groups) and higher redshifts (z>0.5). I'll report on some current progress, future needs and opportunities.



HeCS-SZ: The Hectospec Cluster Survey of SZ-Selected Clusters

Presenter: Ken Rines
Authors: Kenneth Rines, Margaret J. Geller, Antonaldo Diaferio, and Ho Seong Hwang


         Masses of galaxy clusters can be inferred by several independent techniques. Comparing mass estimates from different methods can reveal potential systematic uncertainties in the relation between cluster mass and commonly used observable mass proxies. The recent availability of an all-sky catalog of clusters selected from Planck observations of the Sunyaev-Zeldovich (SZ) effect motivated us to obtain spectroscopic observations of a complete, SZ-selected sample of clusters. We present the first results from HeCS-SZ, a spectroscopic survey with MMT/Hectospec of galaxy clusters selected by their SZ signal. The project builds on earlier samples (CIRS and HeCS) selected by X-ray flux. We measure dynamical masses for 24 additional clusters (using over 8200 new redshifts) to yield a sample of 116 clusters, and a SZ-complete sample of 81 clusters, by far the largest compilation to date. We show that the velocity dispersions scale with the integrated Compton Y parameter according to the scaling expected from X-ray-SZ calibrations. Thus, reconciling the tension between CMB and SZ cosmological parameters requires that both hydrostatic mass estimates and velocity dispersions are significantly biased. The amount of negative velocity bias required is larger than values predicted by simulations. We discuss the impact of X-ray flux selection on the measured scaling relation. Our results provide a test of SZ measurements as cluster mass proxies.



Shaking and stirring galaxy clusters - the devils in the details of the dynamics

Presenter: Elke Roediger
Authors: Elke Roediger, Ralph Kraft, Paul Nulsen, Eugene Churazov, Scott Randall, Christine Jones, Marie Machacek, Ryan Wood

         Cluster mergers and galaxies falling into clusters constantly introduce motions into the ICM. These ICM motions could lead to efficient mixing in the ICM, unless turbulence and fluid instabilities are suppressed by ICM properties such as viscosity or certain magnetic field configurations. Deep X-ray data now reveal the mixing state of the ICM in specific locations, e.g., the wakes of infalling elliptical galaxies, or at sloshing cold fronts. Thus, the observed mixing, or its absence, holds keys to the still ill-constrained ICM transport coefficients. However, an observed mixing state alone does not allow us to draw conclusions about ICM properties -- we need to know 'how much the ICM was stirred’. We report on combining observations and detailed simulations of gas stripping from elliptical cluster galaxies, in particular in the Virgo cluster.



Characterizing the Clusters and Cluster Galaxy Population of redMaPPer Clusters

Presenter: Eduardo Rozo
Authors: Eduardo Rozo


         We will present ongoing work on characterizing the properties of photometrically selected redMaPPer clusters, and present results on the halo occupation distribution of red-sequence galaxies in clusters as constrained from the SDSS redMaPPer clusters sample.



The ICM with a Twist (or two)

Presenter: Lawrence Rudnick


         Motions in the ICM are notoriously difficult to detect, yet they play a critical role in the transport of entropy, metals, and pressure. The imprint of these motions is found in the shapes of cluster radio galaxies, and in the magnetic field structures that can be probed through Faraday rotation. In this talk, I will give an update on work in both of these areas. We can now statistically detect the signature of radio galaxy distortions out to distances of at least 1 Mpc, even for optically selected cluster samples. Better statistics and the use of X-ray or S-Z samples will likely push this to or beyond the virial radius. In addition, many examples of radio galaxy distortions that can only be caused by turbulent-like motions are now being identified in the citizen-science project Radio Galaxy Zoo. I will also give an update on our Faraday Rotation studies of polarized radio emission in and behind Abell 2256. The results are challenging to explain without Faraday contributions local to the radio galaxies, a serious barrier for measurements of the general ICM field structures. This work is supported, in part, by NSF grant AST-1211595 to the University of Minnesota.



Massive molecular gas flows and AGN feedback in galaxy clusters

Presenter: Helen Russell
Authors: Helen Russell, Brian McNamara and Andy Fabian


         Powerful radio jets launched by a central supermassive black hole pump a substantial amount of energy into their host galaxies and cluster environment. This feedback from the central AGN is thought to regulate galaxy growth and cooling of the surrounding hot atmosphere. But many key questions remain, including how the black hole is fuelled, how the heating can be distributed over large scales yet closely coupled to the gas cooling rate and the role of the cold molecular gas apparently cooling from cluster atmospheres. I will present ALMA Early Science observations of molecular gas in central cluster galaxies which show massive outflows driven out by the radio jets and inflows that could be feeding gas disks around the central AGN.



Magnetic Fields and Turbulence in Galaxy Clusters

Presenter: Dongsu Ryu
Authors: Dongsu Ryu


         Clusters of galaxies are filled with hot plasma. Observational evidence and theoretical arguments suggest that the plasma is highly dynamical and probably turbulent, although the properties of turbulence are yet poorly known. The plasma is expected to be permeated with magnetic fields, and then the observed fields are likely the result of turbulence. Here we present a study of magnetohydrodynamic simulations to understand the nature and origin of magnetic fields in clusters. We show that the resulting magnetic fields are sensitive to the nature of forcing as well as the dissipation properties.

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Direct limits on turbulence in clusters using the XMM-RGS gratings

Presenter: Jeremy Sanders
Authors: J. S. Sanders


         The dynamical state of the intracluster medium in clusters of great interest in understanding many aspects of their physics, including their formation, mergers and feedback by active galactic nuclei. We review the results using the RGS X-ray gratings on XMM-Newton to obtain direct upper limits on the velocity width of the intracluster medium in clusters by measuring the line widths. Measured widths include spatial broadening effects, which can optionally be modelled. Although the analysis can only be applied to the cores of the most relaxed objects, we can obtain limits of the order of several hundred km/s.



Dark Matter: Status and Searches

Presenter: Pearl Sandick


         The question of the identity of dark matter is one of the most exciting outstanding puzzles in modern physics. In this talk, I'll describe the variety of tactics currently being pursued to observe particle dark matter, both here on Earth and in the sky, focusing on searches for weakly interacting massive particles (WIMPs). Signals consistent with hints of dark matter have been observed by several experiments. I’ll discuss the implications of these results, and the rapid progress being made to unravel the mystery of the identity of dark matter.


Saro - Poster

Richness-Mass relation and Optical-SZE Positional Offset Distribution for SPT Selected Clusters

Presenter: Alex Saro
Authors: Saro et al.

         We cross-match clusters detected via their Sunyaev Zel'dovich effect in the 2500°^2 SPT-SZ survey carried out with the South Pole Telescope (SPT) with optically selected cluster catalogs extracted from the ~250°^2 science verification data of the Dark Energy Survey: the RedMaPPer and the Voronoi Tessellation (VT) cluster catalogs. We use SPT derived masses and positions of these systems to characterize the RedMaPPer and VT cluster samples. We calibrate the RedMaPPer richness lambda-mass relations for an SPT selected sample and show that it is characterized by low intrinsic scatter sim 18%, consistent with the lamda-mass relation previously estimated by Rykoff et al. (2012), while the resulting VT richness N_(vt)-mass scaling relation is characterized by a larger intrinsic scatter sim 64%. We extend our matching algorithm to SPT selected cluster candidates with lower signal to noise (4 < xi < 4.5) and show that increasing the matched sample by sim 44% with systems that have less well constrained SPT masses only marginally affects a low scatter mass proxy as lambda. We study the offset distribution between the SPT-SZ centre and the optical centers and show that it contains both a dominant, centrally peaked population and a sub-dominant population characterized by larger offsets, consistent with the expected failure rate of identification of brightest central galaxies in the case of the RedMaPPer sample.



A measurement of the kinetic SZ signal towards MACS J0717.5

Presenter: Jack Sayers


         Using resolved two-band SZ images collected with Bolocam, we measure a kinetic SZ signal towards one of the merging sub-clusters of MACS J0717.5. Our observed kinetic SZ signal implies a line of sight velocity of ~3000 km/s for this sub-cluster, in good agreement with spectroscopic measurements of the individual galaxies. In addition to the Bolocam SZ data, our result also relies on sub-millimeter measurements of the background galaxies from Herschel-SPIRE and an X-ray-derived electron temperature from Chandra and XMM. I will describe the details of our multi-wavelength analysis, including a description of the possible systematics and a breakdown of the error budget to quantify what limited our sensitivity to the kinetic SZ signal. I will also provide an update on our ongoing project to use similar data and analysis methods to constrain the kinetic SZ signal in a larger sample of 10 clusters.


Shi - Poster

Astrophysical origin of non-thermal pressure in the ICM

Presenter: Xun Shi

         Non-thermal pressure from random motions in the gas has been found to be non-negligible in hydrodynamical simulations of the intracluster medium (ICM). Possible contribution from magnetic field and cosmic ray ions is also expected from theory. This existence of non-thermal pressure in the ICM biases the cluster mass estimates based on the assumption of hydrostatic equilibrium, and affects the interpretation of observations at cluster outskirts. Therefore, we would like to understand the astrophysical origin of the non-thermal pressure, and be able to predict it quantitatively. I will discuss possible scenarios of how the non-thermal pressure is generated, and present an analytical model for it.



Multiwavelength Analysis of CLASH Clusters

Presenter: Seth Siegel
Authors: S. Siegel, A. Mahdavi, J. Sayers, S. Golwala, M. Donahue, J. Merten, A. Zitrin, M. Meneghetti, K. Umetsu, M. Postman, N. Czakon, T. Downes, P. Koch, K. Lin, S. Molnar, L. Moustakas, T. Mroczkowski, E. Pierpaoli, and J. Shitanishi


         We report on the joint analysis of X-ray, weak lensing, strong lensing, and Sunyaev-Zel'dovich (SZ) measurements of the CLASH sample of 25 massive galaxy clusters. The high-quality multiwavelength data is able to constrain more realistic parametric models for the distribution of dark and baryonic matter in the galaxy clusters, avoiding biases in mass estimates that result from the simplifying assumptions of spherical symmetry and hydrostatic equilibrium. We present results from an initial spherical analysis of a subsample of the clusters, including constraints on the level of non-thermal pressure support in the intracluster medium (ICM). We also discuss preliminary measurements of the shape and orientation of the dark matter and ICM based on a triaxial extension of the model.



Metal abundances in the outskirts of galaxy clusters

Presenter: Aurora Simionescu


         I will present results from Suzaku Key Project observations of the outskirts of the Perseus and Virgo clusters of galaxies. Observations of these bright, nearby targets have allowed us to start probing the chemical enrichment of the intra-cluster medium far beyond the cluster cores and up to their virial radii. I will discuss the intricate interdependence between the temperature structure and metallicity in the cluster outskirts, as well as the conclusions regarding the physics and chemistry of the ICM at large radii that can be drawn robustly from the current data.



The Dark Sky Simulations: Modelling Galaxy Cluster Surveys

Presenter: Samuel Skillman

         The Dark Sky Simulations are a suite of high-resolution cosmological N-body simulations of collisionless matter. We report on the main galaxy cluster results from our largest two simulations, ds14_a and ds14_b, each using 1.07 trillion particles in an 8 Gpc/h and 1 Gpc/h comoving volume, respectively. We use ds14_a to construct an all-sky lightcone to z=2.3 that resolves 10^{13.5} Msun/h halos with 800 particles, providing a unique resource to the community for comparisons with current and future all-sky surveys. We report on discrepancies at the very massive tail of the mass function above 3e15 Msun/h, and discuss their impact on expected number counts from X-ray and Sunyaev-Zel'dovich effect surveys. Using ds14_b, we report on two-point correlation functions of mass scales from ~10^{10} Msun/h to a few 10^{15} Msun/h in a cubic Gpc/h volume. These simulation data products are open access and we describe our novel data access methods that enable interactive analysis of petascale cosmology simulations across the world wide web from a laptop. For more information and data access, see http://darksky.slac.stanford.edu.



Particle acceleration and magnetic field generation in intercluster plasmas

Presenter: Anatoly Spitkovsky
Authors: Anatoly Spitkovsky

         I will review the progress in modeling shock structure and particle acceleration using kinetic simulations, and focus on the current understanding of magnetic field amplification mechanisms, angle dependence of particle injection and the physics behind electron to ion ratio in shock acceleration. I will also describe new scenarios of generation of first magnetic fields during structure formation which can be relevant for the magnetization of plasma in galaxy clusters.


Stanchfield - Poster

High Angular Resolution Imaging of The Sunyaev-Zel'dovich Effect with MUSTANG-2

Presenter: Sara Stanchfield

         MUSTANG-2 is a 90 GHz bolometer camera built for use as a facility instrument on the 100 m Robert C. ByrdGreen Bank radio telescope (GBT). MUSTANG-2 has a 223 element focal plane array currently populated with 64 dual-polarization sensitive transition edge sensor (TES) detectors. The TES detectors are read out using frequency-domain multiplexed SQUID electronics. MUSTANG-2 has significantly improved mapping speed and sensitivity over the original MUSTANG. As a continuum instrument on a large single dish MUSTANG-2 has a combination of high resolution in addition to good sensitivity to extended emission which make it very competitive for a wide range of galactic and extragalactic science. We present the recently completed commissioning results and early science of MUSTANG-2.

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Trasatti - Poster

Radio observations of non-thermal emission from galaxy clusters

Presenter: Monica Trasatti
Authors: M.Trasatti, G. Giovannini, M. Murgia, F. Govoni


         The detection of diffuse synchrotron emission in an increasing number of galaxy clusters helps in unambiguously prove the presence of relativistic electrons and weak magnetic fields in the intra-clustermedium (ICM), together with the hot thermal plasma emitting X-rays.Even though the existence of such sources has been known for three decades, their origin is as yet debated. Their Mpc size requires some form of in-situ production or (re-)acceleration.Their occurrence mostly in un-relaxed clusters suggests that cluster mergers play a key role in producing them.The proposed models differ in the prediction of the spectral behavior of such sources and can be tested through detailed studies of the integrated spectrum and of the spectral index distribution. All these ingredients make the study of these sources not only relevant in terms of their physics, but also crucial for a comprehensive description of the composition and dynamics of the ICM,for the study of particle-acceleration mechanisms as well as for a global understanding of the mechanisms at play during the processes of cluster assembly in the Universe.I will show the status of recent JVLA observations of selected galaxy clusters such as, for example, the under-luminous X-ray clusters A523 and A1213.



ALMA's view of Cold Gas in Cool Cores

Presenter: Grant Tremblay

         New ALMA Cycle 1 observations of the central brightest cluster galaxy in the cooling flow cluster Abell 2597 reveals 100 million solar masses of cold molecular gas entrained in a jet-driven outflow along its central 10 kpc compact steep-spectrum radio source. The outflow is highly multiphase, containing not only cold molecular gas, but also warm ionized and hot X-ray emitting gas, enabling a calculation of the relative mechanical coupling efficiency between a radio jet and all temperature phases of the ambient ISM through which it propagates. The northern radio lobe has also excavated a kpc-scale molecular cavity whose rim is cospatial with an FUV-bright arc of possibly jet-triggered star formation. Strong continuum absorption is seen against the millimeter and radio core where the AGN resides, suggesting that the black hole accretion reservoir is fed by a ballistic rain of ten million solar mass clouds falling at ~400 km/sec into the inner hundred pc of the galaxy’s core. The results show that mechanical AGN feedback, invoked to inhibit cooling flows over a significant fraction of cosmic time, acts not only on the hot X-ray and warm ionized ISM phases, but on the cold molecular phase as well. Moreover, a substantial component of the gas funneling into the black hole accretion reservoir appears to be molecular, inconsistent with the “hot mode” Bondi-like scenario presumably associated with cooling flows that stem from the ambient X-ray halo.


Tudorica - Poster

Galaxy clusters and weak lensing magnificationGalaxy clusters and weak lensing magnification

Presenter: Alexandru Tudorica
Authors: A. Tudorica, H. Hildebrandt

         The growth of large scale structure is one of the fundamentalpredictions of any cosmological model. Galaxy clusters are the highestpeaks in the cosmological matter density field and therefore of primeimportance in cosmology. The calibration of the high-redshift (z>1)galaxy clusters mass-richness relation is particularly important as itcontains information about galaxy clusters in their assembly phase,when assumptions such as virial/hydrostatic equilibrium might not be valid.Measuring the mass-richness relation over a wide range inredshift will help to better understand the astrophysics of clusters overtime and simultaneously provide cosmological structure growthconstraints. An independent and novel method to acquire thisinformation is the use of the weak gravitational lensing magnificationeffect, which is able to accurately measure the masses of largesamples of high-z clusters in a statistical way (i.e. throughstacking) without the need to resolve background galaxies. Thismagnification effect leads to an increase of source counts which thencan be analyzed by measuring the angular cross-correlation function ofoptically selected Lyman-break galaxies to high-redshift clusters. Weapply this method to the hundreds of new high-z galaxy clusters foundin the SpARCS (Spitzer Adaptation of the Red-Sequence Cluster Survey)infrared survey, observed also with the CFHT in the opticalugrz-bands. I will present the Weak Lensing Magnification method and show the resulting mass measurements for a sample of ~900 galaxy clusters over 25 square degrees of SpARCS data, covering a redshift range from 0.1 to 1.9. The mass-richness relation of these SpARCS clusters is compared to literature estimates at lower redshift.

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Suzaku Search for Dark Matter Emission Lines in the X-Ray Brightest Clusters

Presenter: Ondrej Urban
Authors: O. Urban, N. Werner, S. W. Allen, A. Simionescu, J. Kaastra, L. Strigari


         Recently, multiple authors claimed to have detected an emission line in XMM-Newton and Chandra observations of galaxies and clusters of galaxies, that is claimed to have no known atomic line counter part and that could possibly come from decay of sterile neutrinos, a class of dark matter (DM) particle candidates. We performed an analysis of Suzaku observations of the four X-ray brightest galaxy clusters (Perseus, Virgo, Coma and Ophiuchus) to test this potentially exciting discovery. Using an optimized energy range (3.2-5.3keV) and a baseline plasma emission model that incorporates the abundances of elements with the strongest expected emission lines at these energies (S, Ar, Ca) as free parameters we found evidence for an unidentified emission line in the spectra of the Perseus Cluster, whose properties are broadly consistent with the previous claims. Assuming a DM decay origin for the Perseus emission features allows us to predict the properties of this line for the other clusters in our sample. Critically, we do not detect an emission feature at the predicted energy and line flux in the Coma, Virgo and Ophiuchus clusters. These results disfavor a decaying DM interpretation. Further spectral modelling, that allows the abundances of additional elements (Cl, K, Ti and V) as free parameters, suggests, that systematic effects associated with modeling the complex projected spectra for the Perseus Cluster core, details of the assumed ionization balance, and errors in the predicted emissivities of individual spectral lines may in part be responsible for the unidentified emission feature. In the talk I will summarize the modelling techniques and discuss the results of the analysis.



Intermittent Energy Dissipation in MHD Turbulence and Radiative Magnetic Reconnection in the ICM

Presenter: Dmitri A. Uzdensky
Authors: Dmitri A. Uzdensky


         Hot gas in galaxy clusters is a rich natural laboratory of turbulent plasma processes in the weakly collisional regime. In particular, its thermodynamics is influenced by a nontrivial interplay of plasma heating due to the dissipation of magnetohydrodynamic (MHD) turbulence, heat transport by anisotropic thermal conduction, and cooling by bremsstrahlung radiation. In this talk I will review the recent progress in our understanding of the intermittent character of turbulent MHD energy dissipation - and, in particular, of the role played by intense isolated reconnecting current sheets, - and will discuss the implications of these insights for the intracluster medium.

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van der Burg

Evidence for the inside-out growth of the stellar mass distribution in galaxy clusters since z=1

Presenter: Remco van der Burg
Authors: Remco van der Burg, Henk Hoekstra, Adam Muzzin, Cristóbal Sifón, Michael Balogh, Sean McGee


         Measurements on the radial distribution of galaxies in clusters show how well galaxies trace the underlying dark matter distribution, and provide constraints on the physics related to galaxy evolution in these environments. I will present measurements of the spatial distribution of stellar mass in 10 rich clusters from the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) around redshift z~1 (vdBurg+14), and compare this to a similar measurement of their descendant population of 60 clusters at redshift z~0.15 (vdBurg+15). Since the two samples are matched in halo mass to be progenitors/descendants, we can compare their stellar mass distributions directly to learn how the stellar mass content assembles in these clusters. This suggests that the central parts (R<0.4 Mpc) of the stellar mass distributions of satellites in local galaxy clusters are already in place at z~1, and contain sufficient excess material for further BCG growth. Evolving towards z=0, clusters appear to assemble their stellar mass primarily onto the outskirts,making them grow in an inside-out fashion.


van Weeren

LOFAR Observations of Merging Galaxy Clusters

Presenter: Reinout van Weeren


         In a growing number of merging galaxy clusters diffuse extended radio emission has been found, so-called radio relics and halos. Radio relics are mainly found in galaxy cluster outskirts, while halos are centrally located. The existence of these relics and halos implies the presence of relativistic particles and magnetic fields in the intracluster medium. The main question is how these relativistic particles are accelerated and the origin of cluster-wide magnetic fields. Halos and relics have steep synchrotron spectra and are thus best studied at low frequencies. In this talk I will present the latest result from LOFAR, a revolutionary new radio telescope which enables studies of diffuse cluster radio emission with unprecedented resolution and sensitivity.



Radio Observations of Galaxy Clusters

Presenter: Tiziana Venturi
Authors: T. Venturi


         I will provide a general introduction to the diffuse radio emission whichmay be found in clusters of galaxies, with special attention to giantradio halos and mini-halos. These are diffuse synchrotron sources which permeate the intracluster medium.I will report on the final results of the Extended GMRT Radio Halo Survey,tailored to address the question of the occurrence of radio halos in theredshift range 0.2-0.4 (a total of 65 clusters). The survey, started in 2005 and just completed, led to the discovery of a number of new diffuse sources, and highlighted the close connection between giant radio halos and mini-halos and the cluster dynamics, providing support to the turbulent re-acceleration model for the formation of giant radio halos. The propertiesof the syncrothron spectra will be shown and discussed.Finally, I will report on GMRT observations of the nearby galaxy cluster A2142, which shows exceptional thermal and non-thermal properties. It hosts two central cold fronts, and a third one, at the distance of ~ 1 Mpc from the cluster centre. Moreover, it shows evidence of group accretion, with a prominent tail of X-ray emission associated with an infalling group. I will report on the giant radio halo imaged with the GMRT and will briefly discuss possible interpretations for its origin.


Vijayaraghavan - Poster

The Dynamical Evolution of Galaxies and their Gas in Cluster Environments

Presenter: Rukmani Vijayaraghavan
Authors: Rukmani Vijayaraghavan, Paul RIcker


         Galaxies in cluster environments tend to be gas-poor and lack active star formation in comparison with field galaxies. Some of these differences arise from environmental influences such as ram pressure and tidal stripping. We investigate these influences using N-body simulations of galaxies within idealized cluster cluster merger environments. Using these simulations, we study the velocity distributions and phase-space structure of infalling group galaxies as they virialize within their host cluster and relate them to the observed dynamics of Virgo dwarf elliptical galaxies. We additionally use N-body plus hydrodynamic simulations to study the stripping of hot gaseous coronae from galaxies in groups and clusters. We use these simulations to characterize ram pressure stripped gas tails and their dependence on galaxy mass and environmental properties. We also generate mock X-ray observations of the simulated galaxies and use a stacking analysis to evaluate the detectability of the remaining gas.



Precipitation and the "Copenhagen Interpretation" of Feedback

Presenter: Mark Voit
Authors: G M Voit


         My talk will present a framework for interpreting how AGN feedback regulates the evolution of central cluster galaxies. Many pieces of that framework were discussed at last summer's ICM Theory and Computation workshop in Copenhagen. The key idea is that cluster cores are regulated by a precipitation-driven feedback mechanism, in which cold clouds precipitate out of the intracluster medium via thermal instability and subsequently accrete chaotically onto the central black hole. I will present compelling observational evidence in support of this mechanism in both galaxy cluster cores and isolated massive elliptical galaxies. While it may be presumptuous to call the overall framework a "Copenhagen Interpretation," we have also found that precipitation-driven feedback can account for many other properties of galaxies, from central cluster galaxies all the way down to Local Group dwarfs, indicating that the principles at play in cluster cores might be fundamental for all of galaxy evolution.

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Exploring the outskirts of galaxy clusters

Presenter: Stephen Walker
Authors: Stephen Walker


         Suzaku has allowed a breakthrough to be made in the study of the low surface brightness X-ray emission from the outskirts of galaxy clusters, providing exciting and unique observations of these previously unexplored regions of the intracluster medium. In the outskirts, gas is continuing to accrete onto clusters, allowing us to see the formation process in action and to probe the physical processes in the ICM near the virial radius. We present results from our observations of the outskirts of the nearby Centaurus cluster using both Suzaku and Chandra, and also results from our search for large scale gas sloshing in galaxy clusters. We also present preliminary results from our latest Suzaku observations. The general picture of galaxy cluster outskirts which has emerged from the catalogue of clusters observed with Suzaku so far will be discussed.



Deep Chandra observation of the nearest cold front in the sky - implications for the microphysics of the hot intracluster plasma

Presenter: Norbert Werner

         I will present the results of a very deep (500 ks) legacy-class Chandra observation of the nearest, best-resolved cold front in the sky which lies 90 kpc (19') to the north of M87. The observation provides an extraordinary improvement in resolution (resolving scales smaller than 80 pc), allowing us to study the cold front interface in unprecedented detail.



NuSTAR Observations of Galaxy Clusters

Presenter: Daniel Wik
Authors: D. Wik, S. Molendi, A. Hornstrup, F. Gastaldello, N. J. Westergaard, G. Madejski, F. Harrison, and the NuSTAR team


         Efforts to characterize the hard (>10 keV) X-ray emission from galaxy clusters have historically been hampered by a lack of observatories with imaging capabilities. As the first orbiting observatory with mirrors able to focus >10 keV X-rays, NuSTAR makes possible the most sensitive searches yet for inverse Compton emission associated with radio halos and relics and tight constraints on the temperatures of shocked regions. I will present highlights from the galaxy clusters so far observed by NuSTAR, including the Bullet Cluster, the Coma Cluster, the Cygnus A Cluster, and Abell 2256. In particular, I will spotlight spatially resolved, joint spectral fits to ~414 ks of Chandra and 266 ks of NuSTAR exposures of the Bullet Cluster. These fits constrain the temperatures of the hottest (~20 keV) shocked gas in the cluster, and I will discuss its implications on the thermalization and dynamical state of the intracluster medium.



The XXL survey

Presenter: Jon Willis
Authors: J. Willis, M. Pierre and the XXL collaboration


         The XXL survey is an XMM Very Large program which covers two 25 deg2 fields at medium X-ray sensitivity. The survey has detected over 400 clusters and 30,000 AGN to a flux limit of 5e-15 cgs in the soft band. The survey aims to provide a sensitive test of our cosmological model by measuring the growth of structure as traced by massive clusters. Primary X-ray observations were completed in 2013 and we will describe the XMM data set in addition to our multi-wavelength follow-up campaign. We will also summarise the main science themes engaged by the XXL survey including the computation of a realistic selection function and a series of early results generated for the sample of 100 brightest clusters detected in the survey.



Merging Cluster Collaboration: Dynamics of 25 Radio-Selected Mergers

Presenter: David Wittman
Authors: David Wittman, Will Dawson, Nate Golovich, Merging Cluster Collaboration


         We present initial results on a spectroscopic and weak lensing survey of 25 merging systems selected on the basis of their radio relics. Based on ~200 galaxy member spectra per system, we assess the number of subclusters and measure the rest-frame relative line-of-sight velocity of the two main subclusters in each system with a precision of ~100-150 km/s. We find that v_los is quite small, typically a few hundred km/s and usually consistent with zero. This demonstrates that radio relic selection is efficient at discovering transverse mergers (i.e., systems with merger axes nearly in the plane of the sky), and/or systems near turnaround; we argue that transverseness is the dominant effect. Weak lensing masses of these systems are quite high, indicating that radio relic selection is efficient at picking up the most massive mergers. Massive mergers without radio relics are likely to be too young to have accelerated electrons sufficiently; preliminary results of dynamical modeling show that our systems are relatively old compared to, e.g., the Bullet Cluster. This ensemble of systems will become a laboratory for testing dark matter interactions, merger effects on galaxy evolution (including triggering or quenching of star formation), and the physics of the shock and particle acceleration.



Cluster mass cross-calibration: the covariance between hot gas and galaxies in clusters

Presenter: Hao-Yi Wu
Authors: Hao-Yi Wu, August E. Evrard, Oliver Hahn, Davide Martizzi, Romain Teyssier, Risa H. Wechsler


         The combination of hot gas and stellar mass measurements of clusters are essential for calibrating the total mass of clusters. In this talk, I will introduce a new set of hydrodynamical simulations “Rhapsody-G” and present a new calibration of the scatter and covariances of gas and stellar properties of clusters. I will present the discovery of several covariances between gas and stellar properties, and how these covariances help us improve cluster mass cross-calibration. I will also discuss how galaxy clusters closely resemble closed boxes even at non-linear scales.

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Yang - Poster

The Interplay among Cooling, Thermal Conduction and AGN Feedback

Presenter: H.-Y. Karen Yang
Authors: H.-Y. Karen Yang & Chris S. Reynolds (U of Maryland)

         Feedback from central active galactic nuclei (AGN) is one of the most promising heating mechanisms that may halt catastrophic cooling of cool-core clusters. However, this picture is complicated by the fact that anisotropic thermal conduction could insulate the cores by field line wrapping due to the heat-flux driven buoyancy instability (HBI). In this work, we investigate the interplay among cooling, thermal conduction along magnetic field lines, and kinetic feedback from AGN by performing 3D magnetohydrodynamic simulations using FLASH. The following questions are addressed: (1) whether the HBI can be suppressed under realistic cluster conditions, e.g., by magnetic tension or by AGN-driven turbulence, (2) how thermal conduction affects the properties of AGN feedback, and (3) what the relative importance of mechanical and conductive heating is within the cool cores.

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Gamma-ray Observations of Galaxy Clusters

Presenter: Fabio Zandanel


         Cosmic-ray (CR) protons are confined in galaxy clusters and can accumulate there for cosmological times, where hadronic interactions with the intra-cluster medium can generate secondary electrons, gamma-rays and neutrinos. I will briefly review gamma-ray observations of galaxy clusters with focus on the latest results that push the CR-to-thermal pressure down to (sub-)percent levels, and strongly constrain the CR acceleration efficiency at shocks. I will finally discuss the broader implications for the non-thermal emission from hadronic interactions in clusters.



Turbulent heating in the X-ray brightest galaxy clusters

Presenter: Irina Zhuravleva


         Radiative energy losses of the hot gas in galaxy clusters occur on timescales significantly shorter than the Hubble time, leading to massive accumulation of cold gas and vigorous star formation, in contradiction to observations. Several sources of heat have been discussed, most promising being heating by the SMBH in central galaxies through inflation of bubbles of relativistic plasma. The missing link in this scenario is the mechanisms, by which energy from bubbles is transported to the hot gas. Dissipation of gas turbulence, induced by bubbles during their buoyant rise and expansion, is a possible mechanisms. However, direct measurements of gas velocities will be possible only with future X-ray calorimeter on board the Astro-H observatory. We recently overcame this problem, by analyzing long Chandra observations of the X-ray brightest clusters of galaxies and measuring statistical properties of density fluctuations, which allowed us to constrain the velocity power spectrum of gas motions in the ICM indirectly. I will show that the heating rate due to dissipation of turbulence is indeed sufficient to balance the radiative cooling locally at each radius within the cores. Turbulent dissipation, therefore, might be the key element in resolving the gas cooling problem in cluster cores and other X-ray gas-rich systems.



Galaxy Cluster Gas Motions and Astro-H: Predictions and Challenges from Simulations

Presenter: John ZuHone
Authors: John ZuHone, Mark Bautz, Maxim Markevitch, Aurora Simionescu, Eric Miller, Irina Zhuravleva


         The Astro-H satellite will be the first instrument to be capable of elucidating the velocity structure of galaxy cluster plasmas via measurements of the shift and width of spectral lines. A number of interesting nearby clusters have been proposed as targets for the mission, including objects undergoing sloshing, turbulence, and major mergers. In this talk, I will present predictions from hydrodynamical and magnetohydrodynamical simulations for the effects of these various types of gas motions on the shift and shape of spectral lines, and the resulting implications for determining the velocity structure and the detailed physics of the intracluster medium.

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