GALAXIES: PUTTING IT ALTOGETHER

Galaxies can appear pathological in their shapes or that their gas is distributed in odd patterns. This is due to interactions between galaxies; we sometimes are seeing the galaxy well after a collision, and pathological shapes suggest that an interaction happened in the past. If two galaxies pass close enough to one another, they can become gravitationally bound and eventually merge together. When galaxies merge, the stars actually never collide, only the gas collides. Thus, the gas usually falls to the center or the merging system while the stars stream in what are known as tidal tails. When the gas collides, it forms new stars.

The bottom line of this lecture is that the Monolithic Collapse model of the 1960s is wrong. Galaxies do not form by the collapse of an isolated gas cloud. This also means the Hubble Tuning Fork version of galaxy evolution and formation is wrong.

We now belive galaxies form in the context of the cosmic web and by the violent merging of smaller galaxies into larger galaxies. The below movies illustrate these processes quite well. Enjoy!

Movie: The Mice
Click here to watch

This movie was motivated to help understand the merging system known as The Mice. To set up the simulation, the galaxy masses, rotation directions and speeds, and approach vectors had to be estimated. Also, the viewing angle had to be chosen. So, there is a bit of educated guess work, but if the simulation accurately models the merging system, then it gives us confidence that these input quantities correctly characterize the system. This helps us understand the physics of merging galaxies. In this case, not only do we learn the initial conditions of the merger, but we also deduce that The Mice are infalling and on their second passage with each other!

When playing the movie, try to estimate when the simulation matches The Mice. In practice, astronomers analyze the detailed motions of the stars in The Mice and quantitatively compare them to the motions of the stars in the simulations. It is very detailed business.

Movie: Galaxy Evolution and Formation
Click here to watch

A movie of a cosmological simulation. First, the dark matter is shown (whitish). Note the cosmic web of filaments and the concentrations that are at the intersections of the filaments. When the simulation pauses and the image rotates the scale across the image is about 20 million light years! On this scale, individual galaxies are pin points, whereas galaxy clusters are the knots at the intersections of the filaments. Then the movie switches to green, the gas (normal matter) and evolves in time showing how the gas dynamically concentrates. Then the simulation shows the stars, including the eventual formation of a Milky Way like galaxy. Note how this teaches us how our own galaxy was formed.. and it is a violent process! Finally, we see the fully flowered model galaxy.

Movie: Cosmological Distribution of Metals
Click here to watch

This is a movie of how the metals (created in stars and then distributed back into and beyond the galaxies) evolve with cosmic time. The left panel is the total density of metals (cm^-3) and the right panel shows how the low ionization (traced by CII ions), intermediate ionization (traced by CIV ions), and high ionization (traced by OVI ions) evolve around galaxies and in the intergalactic medium. The panels are roughly 20 Mpc on a side. On this scale, the galaxies themselves are pinpoints, so you are looking at a huge volume of the universe (about 20 Mpc across). The simulation begins at z=20 and evolves to z=0 (present time). Note that the low ionization gas (red) is located in small halos coinciding with the galaxies and that the intermediate ionization gas (green) is in larger more extended halos surrounding the low ionization halos. The highest ionization gas (blue) grows dramatically throughout the intergalactic medium around redshift 3 to 2. This is because (and this is included in the simulation), QSOs were very abundant during this brief period of the universe and the QSOs send out a powerful background of high energy photons which highly ionizes the diffuse low density intergalactic gas.