Expanding the horizons needs methods to establish distance. The size of the Milky Way Galaxy was established when the less luminous but RR-Lyrae stars were used by Harlow Shapley. At that point the Sun was also pushed out of the center of the galaxy. It is now known to be about 8kpc away from that center. The diameter of luminous matter in the Milky Way galaxy is about 30kpc.
Later the more luminous Cepheids in Andromeda were used to measure the distance to Andromeda. An original error in that distance measurement was corrected when it was established that there are two different classes of Cepheids. All of these variable star classes lie in the instability strip of the Hertzsprung-Russell diagram. Remind yourselves how luminosity helps to determine distance.
The Milky Way has a couple of distinct components: a thin disk, a thick disk, a central bulge, and a halo. The galactic center lies in the bulge. Both the thick disk and the halo are old: they contain only long lived stars; the short lived ones have died already. The halo also contains the old globular clusters. As one moves out perpendicular to the disk, the stars generally grow older as it took them longer to move away from the thin disks starforming regions. The extent of the thick disk cannot be understood though by just assuming that the old stars that the thick disk is made from slowly moved out of the thin disk. That assumption is only true for the innermost part of the thick disk.
Only the bulge and the thin disk contain gas and dust: new stars can only be formed there. So they are the only parts that contains young stars and short lived very luminous B and O stars.
Most of the mass of the Galaxy is in gas and dust, and most of the gas is Hydrogen. Hot ionized Hydrogen emits H-II radiation, and cold atomic Hydrogen emits H-I radiation. From the Hydrogen H-I and H-II emission lines we know about the spiral structure of the Milky Way galaxy. As any direction in the plane will always cut through more than one arm, we need a model of the galaxy and its rotation to interpret the various measured redshifts in that direction. Once the model reproduces all measured redshifts, we see the spiral structure in the distribution of both hot and cold Hydrogen.
The motion of stars in the disk is ordered as they follow their orbitals around the galactic center. Perturbations to that ordered motion arise from the mutual gravitational attraction of the orbiting stars (and dust). Globular Clusters and stars in the halo have no such ordered orbitals. They orbit the galactic center in random directions not aligned with the general rotation of the stars and gas in the disk.
The differential rotation of matter (stars and gas) in the galactic disk is so fast that it would erase the spiral structure in short time. While we cannot be sure, the best theory is that pressure waves in the interstellar medium that do move slower than the overall rotation of the disk are responsible for the spiral structure we see in our own galaxy as well as other spirals. The arrival of this pressure wave would also trigger star formation, and indeed we find the most luminous short lived stars inside the spiral arms, i.e. the pressure wave.
The so-called rotation curve for our galaxy reveals a large amount of dark matter (DM): matter that gravitates but does not emit or absorb any light... Galactic rotation curves for other galaxies show the same phenomenon; generally about 3-10 times as much mass is in the DM halo of a galaxy as there is in its luminous matter. Microlensing experiments show that this matter is not just hidden in very low luminosity stars or non-star Jupiter like objects where we would not find it in our telescopes. At the center of our galaxy there is a 3.6 billion solar mass Black Hole (BH). We infer its mass from the measured orbits of the innermost stars orbiting around this supermassive BH.
The Milky Way and Andromeda are likely on a collision course. We know the radial component of their relative motion very well, but not the proper motion component of the two. In the lecture we saw a simulation of that collision and then some examples of colliding galaxies as observed through our telescopes. In galaxy collisions the stars in either galaxy normally do not collide; the space between them is to vast. Tidal forces though dramatically change the shape of both (equally large) galaxies.
Galaxy classification uses four distinct categories: Ellipticals, Spirals, barred Spirals, and Irregulars. Galaxies exist that have properties of both: the disk of a Spiral, and the outside shape of an Elliptical; these are sometimes referred to as lenticular galaxies. Hubble's Tuning Fork cannot be seen as representing the evolution of galaxies in any way.
The size (extent) of the bulge further classifies the spiral galaxies, both barred and non-barred. Sa is the classification for a Spiral with a big bulge, whereas SBc denotes a barred Spiral with a small bulge. Ellipticals are classified according to their shape: E0 is a round Elliptical, E5 denotes an elongated (elliptical) Elliptical. This classification scheme does not take into account our perspective: an Elliptical that appears round from Earth may very well be quite elongated if viewed from a very different angle.
The largest and the smallest galaxies are Elliptical galaxies. The largest Ellipticals are thought to be the result of galaxy mergers: The merger simulation between the two Spirals Milky Way and Andromeda in the end yielded an elliptical shape. Ellipticals do not have a disk (as is expected if they were formed from the collapse of a gas cloud) and contain no cold gas that could be used for star formation. So they contain mostly old reddish stars. That is particularly true for the smallest Ellipticals; those may very well be the oldest galaxy population of the universe. Star movement in Ellipticals seems mostly random; there is very little overall rotation of the galaxy as a whole.
Spiral galaxies contain dust and starforming regions with the ensuing populations of young stars; they are not necessarily young themselves though. Like large Ellipticals they gravitationally attract and swallow smaller galaxies, growing in mass. In the process of being absorbed the smaller galaxy may pass through the disk a couple of times just like the globular clusters do; the Cartwheel galaxy is seen as an example of how this process triggers star formation in the dust of the spirals disk, witnessed by a rim of short lived but luminous O and B stars (young stars) where the smaller satellite galaxy seems to just have passed.
Irregular galaxies are just that: irregular in shape. This is likely due to the fact that they have recently undergone a galaxy collision.
Distances to spiral galaxies can be estimated with the help of the Tully-Fisher relation. The orbital speed of stars in a galaxy measures the total mass contained in the orbit (rotation curve). Rotation leads to broadening of spectral lines; in particular the 21cm emission line of atomic Hydrogen. The mass of a galaxy is related to the number of stars it contains, and the number of stars determines the amount of light it emits: a galaxy's luminosity. The Tully-Fisher relation relates the rotational broadening of galactic emission to the luminosity of a spiral galaxy. Again we can use our estimate of a galaxy's luminosity to measure a galaxies distance.
Galaxy are grouped in clusters that themselves tend to group into superclusters. Only maybe 20-30% of all galaxies are not a member of a group but move through space alone.
Using Tully-Fisher distances and distances derived from supernova type 1 (SN-1)
explosions in distant galaxies we can see that a galaxy's redshift increases
with distance. This so-called Hubble's Law is used to estimate the distance
to the furthest galaxies. The expansion of space in the universe itself
leads to this redshift; it is not a recession redshift: if the Earth and
some remote galaxy were not moving with respect to each other at the time that
the line was emitted, the expansion of space during its travel to earth will
redshift it! This is different from redshift arising from the relative motion
at the time of emission.
Later comparison of redshift for SN-1 with their distance
estimated from intrinsic luminosity will lead to conclusions about
the history of the expansion of the Universe.
Please review the cosmic distance ladder!
Irregular galaxies are irregular in shape; other galaxies are unusual not by shape but by their energy output characteristics. Normal galaxies (spiral, elliptical, lenticular, or irregular) emit a "stellar" spectrum, i.e. a spectrum determined by the stars in the galaxy. The energy put out by galaxies with a non-stellar spectrum is typically much larger than expected from the visible ("stellar") part of the spectrum. Such galaxies are often seen in the radio or X-ray wavelength (or both) where normal galaxies do not emit much radiation. For all of these galaxies we nowadays assume a single mechanism that explains the energy output: A supermassive BH at the center with matter falling into it. The gravitational energy released from matter falling into the the BH powers these galaxies' enormous energy output. The energy is bundled into two opposing jets that emerge from opposite sides of the central BH. If these jets are stopped inside the bulge of the galaxy X-ray emission is seen from the region where these jets heat up the material in the bulge. If the jets escape the BH's galaxy they can stretch distances comparable to the dimensions of our whole local group of galaxies. Whether we catalog these galaxies as Quasars, Blazars, Seyfarts, or Radio Galaxies of various types only depends on the angle under which we look at the jets.
Statistical analysis of intra-cluster motion of galaxies in rich clusters suggest that while the dark-to-visible-matter-ratio for an individual galaxy is of the order of 3-10, it is of the order 10-100 for galaxy clusters as a whole. Dark matter is quite prevalent on larger scales.
Redshift surveys map the distances to galaxies in an effort to discern structure in the distribution of (luminous) matter. They reveal voids of ~100 Mpc size, bounded by walls, filaments, or sheets of galaxy clusters.
Beyond about 200Mpc there is no discernible structure in the universe.
Distant Quasars provide a strong, pointlike Lyman-alpha source. Absorption features at different redshifts during propagation to Earth can be used to reconstruct the extend and distribution of Hydrogen clouds along the way of this light beam.