1. Large Scale Structure Revisited (again!)

Wide-angle surveys: shallow but broad

vs.

``pencil-beam'' surveys: narrow but deep

Observed: lack of homogeneity in the galaxy distribution

Homogeneous means: the same everywhere

Non-isotropic ?

Isotropic means: looks the same in every direction

Do observations `fly in the face' of Universal Expansion?

The Universe on very large scales is:

neither smooth nor uniform

- large voids (40-100 Mpc)

- clusters and super-clusters (1-40 Mpc)

- The Great Wall

- Structures up to 200 Mpc across!

Compare to total size of observable universe: 4000 Mpc

Large scale structures on scales 5% of this size!

Perhaps only on the largest scales, the Universe looks the same in every direction.

2. 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. 17.6)

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:

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

- stars formed early before the disk formed.

Some general truths

Bulges: stars formed before gas cloud collapse

Disks: stars formed after gas cloud 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.

Most bulges formed early in the Universe

3. 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!

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

From the ground, distant galaxies look like smudges of 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.

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.