1. Galaxy Formation

We do not know for sure how galaxies formed . . .

. . . but we have a lot of ideas!

Recall the scenario for Milky Way's formation (Sec. 14.3.3)

Some similarities with formation of a solar system:

- pre-galactic cloud of gas

- slowly rotating

- slowly collapsing

- conservation of angular momentum leads to formation of a rapidly spinning disk

Some differences with formation of the solar system:

- galaxies (ours and others) may have formed from the combination of several smaller clouds that merged over a long period of time.

- some stars formed early before the disk formed.
(what would be the analogy for the Solar System?)

Some general truths

Disks: stars formed after gas cloud collapse (why?)

Bulges:

• either stars formed before gas cloud collapse, or...
  
• stars in a stellar disk are "heated" in a dynamical sense, i.e., their orbits are re-arranged via tidal disruption (mergers, interactions) or dynamical instabilities
  
  (see a terrestial example: Tacoma Narrows bridge collapse.)

But did bulges form early or late?

How could they form late?

galaxy-galaxy mergers

We currently believe that if this happened, it happened mostly early on in the Universe if the disk today is old. (why?)

Most bulges in disk galaxies formed early in the Universe

...but let's look at a few counter-examples

Some simulations, courtesy of Mattias Steinmetz and collaborators, of structure formation:

• in an expanding universe with Newtonian Gravity on small (1 Mpc) scales;
• with cold, dark matter;
• sticky gas; and
• ad hoc prescriptions for
  

  converting gas into (collisionless) stars,
  recylcing stellar ejecta (chemical and gas-phase evolution), and
  energy feedback from stellar winds and explosions).

  • something from nothing   --   a giant elliptical
  • structure formation on large scales   (10's of Mpc)
  • structure formation on small scales   (1 Mpc)   --   the Milky Way?

Is this what we observe ???

2. Galaxy Evolution

Recall that the Universe is a ``time machine:''

The farther out we look in space the farther back we are looking in time.

So if we don't know how galaxies formed a priori . . .

. . . just go observe it happening!

Here's the Hubble Ultra Deep Field "SkyWalker" -- it doesn't get better than this (so far), but it will.

Easier said than done:

We have observed galaxies to distances of 10 billion light years

- most of the age of the Universe.

But:

The farther away objects are, the fainter and more ``redshifted'' their light.

Can't watch a single galaxy evolve, but have to contrast ``comparable samples''.

However, there are many kinds of galaxies, even in the local Universe.

From the ground, distant galaxies look like smudges of light.

How would you know you are looking at the same sample of galaxies, when observed at different distances, particularly since you are looking for things to be different?

This is the problem.

(a) Stars mostly formed early on.

(b) Stars formed before gas collapsed into a disk.

(c) Originally there were proto-galactic clouds of gas.

(d) Gas dissipation was important in shaping the over-all matter distribution.

(e) Rotation played a large role in regulating star-formation.

(a) Distant galaxies are apparently faint.

(b) Distant galaxies are apparently small.

(c) The light of distant galaxies is redshifted.

(d) It's hard to define comparable galaxy samples at different distances.

(e) We can't find any galaxies at large distances.