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

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

Remember the players: Slide 3

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

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

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)

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)

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:

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.

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.

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!

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

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.

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

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.

Gas can be stripped out in tidal tails Slide 16

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.

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.

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.