The mechanisms by which galaxies evolve underpin some of the most fundamental questions in cosmology — in particular, how the baryonic content of the Universe evolves into the range of galaxies observable across cosmic time. The Milky Way Galaxy serves as a unique "archeological laboratory" in which we can study the signatures of galaxy evolution on the scales of individual stars and small interstellar clouds.

My research focuses on the stars, gas, and dust of the Milky Way, using high resolution spectroscopy, large photometric and spectrocopic data sets, and theoretical models.

In addition, I play a leading role in the SDSS-APOGEE spectroscopic survey, which is revolutionizing our view of the Milky Way and the power of Galactic archeology.

The Milky Way rising over a town.

Stellar Populations

Galactic bulge and disk stellar populations contain most of the evolutionary records spanning a galaxy's history. Our understanding of these (resolved) stellar fossil records in the Milky Way's bulge and disk drives our interpretation of their (unresolved) extragalactic counterparts, but the high extinction and complex morphology of the disk midplane and inner Galaxy preclude a clear understanding of these regions.

I work with large photometric and spectroscopic datasets to characterize the Milky Way extinction and the stellar populations that lie behind it, including open clusters in the outer Galactic disk, the morphology and dynamics of the inner Galactic bar, and the presence of chemical outliers in the bulge. These data hold the key to understanding bulge and disk evolution via the chemodynamical patterns in the intermixed stellar populations.

Interstellar Material

The ISM of a galaxy plays a critical role in its chemical evolution. The ISM transports all of the heavy elements in the disk and bulge (that are not locked up in stars) throughout the galaxy, possibly to be incorporated into later stellar generations.

One of the least understood ISM components comprises carbon-rich large molecules (e.g., PAHs). I developed new techniques for measuring certain absorption features (called DIBs) from these molecules in the spectra of distant giant stars, mapped the density distribution and velocity field of this interstellar phase along tens of thousands of sightlines, and discovered a very close correlation between the DIB at 1.53μ and interstellar dust reddening. This analysis represents a fundamentally unique approach to the ISM by using the DIBs as statistical tracers of Galactic structure and the multi-component ISM.

I have characterized the interstellar dust distribution using infrared stellar photometry, measured the wavelength dependence of the extinction, identified targets for spectroscopic surveys in highly reddened regions, and created 2- and 3-D extinction maps. I used stellar benchmarks with spectroscopic extinction and distance estimates to validate Galactic dust maps and independently identify the presence of large-scale dust features.

Top: The strength of DIB absorption seen in optical wavelengths from SDSS background galaxies and quasars (Lan et al. 2015) and in infrared wavelengths with APOGEE (Zasowski et al. 2015). Click here for an interactive version of this map! Bottom: The motion of the APOGEE DIB molecules with respect to the Sun. Image credit: T. W. Lan and G. Zasowski.

This research is described in a AAS press release and on the SDSS Science Blog. Click here to download a catalog of the APOGEE detections.

APOGEE observing the Milky Way with the Sloan 2.5-meter telescope.
Image Credit: S. R. Majewski

APOGEE: The Apache Point Observatory Galactic Evolution Experiment

APOGEE, part of the current generation of the Sloan Digital Sky Survey (SDSS), is a high resolution, infrared spectroscopic survey of nearly half a million stars in the Milky Way's bulge, disk, halo, and satellites. I am currently the Field & Target Coordinator and an SDSS-IV Architect for APOGEE-2. I helped establish the science goals and requirements, and have led the efforts to define and document the survey target sample and write the targeting pipeline.
Check out APOGEE's first press release here, with a video I helped create!