1. Cosmology and extragalactic astronomy
Mat Page
Mullard Space Science Lab, UCL
13. Theories of galaxy formation and evolution

2. 13. Theories of galaxy evolution
Slide 2
13. Theories of galaxy evolution
This lecture:
Connecting CMB fluctuations to the galaxies we see today
Types of dark matter
How spirals and ellipticals form

3. Remember the players:
Slide 3

4. Where do we start?
Slide 4
Before recombination (380,000 years) universe is opaque.
The photon pressure resists growth of inhomogeneities
At recombination structures start to grow from the seeds in the cosmic microwave background.
Then the way galaxies and other structures form depends more than anything else on dark matter

5. How does the dark matter affect galaxy formation?
Slide 5
How does the dark matter affect galaxy formation?
Hot or cold?
If the particles of dark matter have low velocities (like a cold gas) they will fall under their own gravity and small inhomogeneities will grow.
first structures to form will be small
If the particles of dark matter have high velocities (like a hot gas) they will smooth out inhomogeneities on small scales
first structures to form will be big

6. Hot dark matter “Top down galaxy formation”
Slide 6
“Top down galaxy formation”
biggest things form first
fragment into smallest things
Clusters should be the first things to form
galaxies fragment from clusters
galaxies should be recent phenomenon
(z<1)

7. Cold dark matter “bottom up galaxy formation”
Slide 7
“bottom up galaxy formation”
also known as hierarchical formation
small things form first
combine to make big things.
small galaxies form first
merge together to form larger galaxies
these gather together into groups
accumulate to form clusters.
Clusters should be recent
(z<2)

8. Cold dark matter Big computer simulations now very detailed.
Slide 8
Big computer simulations now very detailed.
Predict the dark matter and so galaxy distribution in space:

9. How do we decide which one is right? Actually it’s quite easy to tell.
Bottom-up or top-down?
Slide 9
How do we decide which one is right?
Actually it’s quite easy to tell.
Remember that in top-down model, galaxies should be recent.

10. But galaxies have been around for ages, many now found at z>3.
Slide 10
But galaxies have been around for ages, many now found at z>3.

11. Ellipticals and discs, which came first?
Slide 11
Scenario 1: monolithic collapse of ellipticals
cloud collapses
single burst of star formation
after star formation finished, small elliptical galaxy
Scenario 2: accretion of a disc
intergalactic gas cloud accreted as disc.
now it is an Sa galaxy!

12. Ellipticals and discs, which came first?
Slide 12
Scenario 3: hierarchical construction of ellipticals
intergalactic gas accreted into dark matter halo
disc galaxy forms
several halos collide and merge
gas is turned into stars
elliptical galaxy

13. Which scenario seems more likely?
Slide 13
Deep Hubble space telescope images give some good insight.
At z>1 there appear to have been many more blue, disc-like and irregular galaxies.
Where have they gone?
Presumably they have merged together to form present day ellipticals.

14. Galaxy interactions and galaxy discs
Slide 14
Galaxy discs are quite fragile.
The tidal distortions seen here...

15. Galaxy interactions and galaxy discs
Slide 15
… can totally spoil the disc as in the antennae.
Look how much star formation this has set in motion!
This is a clue to the fate of this galaxy.

16. Gas can be stripped out in tidal tails
Slide 16

17. Does hierarchical merging work to make elliptical galaxies?
Slide 17
Yes:
loads of star formation induced in the antennae
stripping of gas in tidal tails
the loss of the ordered motion of galaxy discs
must make something like an elliptical galaxy.
However, we will see in the next lecture that there are other ways to make elliptical galaxies.

18. But what doesn’t work?
Slide 18
Key point here is that you can form a disordered bunch of stars from galaxy discs
You cannot form an ordered disc from disordered stars.
Can make ellipticals from spirals by merging them.
Can only make spirals from ellipticals by accreting more gas from intergalactic medium.

19. Galaxy formation: key points
Slide 19
Fluctuations begin to grow through gravity only after recombination.
Cold dark matter produces bottom up hierarchical galaxy formation.
Hot dark matter produces top down galaxy formation.
The observations favour cold dark matter.