Lecture 29: From Smooth to Lumpy Astronomy 1143 – Spring 2014

Key Ideas: The Universe today has very high density regions Gravity pulls matter together High density regions continue to grow; low density regions lose out Dark matter helps density fluctuations grow Nature of dark matter important for the growth of structure Determines the size of collapsed structures Hot (top-down) vs. cold (bottom-up) dark matter Galaxies grow via mergers in CDM Universe

Density of the Universe The average density of the universe is kg/m 3. less dense However, most of the universe is slightly less dense than average (voids). much denser Some of the universe is much denser than average (stars, white dwarfs, black holes…) How did this happen? GRAVITY

Gravity Gravity increases the lumpiness of matter. Long-range force that is always attractive 1234

Movie of the collapse of structure Computer simulation Billions of dark matter particles Move under the influence of gravity Co-moving box = box grows as the Universe expands and we are riding along Structure appears as gravity makes dense things denser Filaments, voids and clusters

Dense regions at the time the universe became transparent have evolved to become clusters & superclusters today. The presence of dark matter is exceeding helpful for gravitational collapse. But dark matter does not cool, so it is more spread out than baryonic matter in the present Universe.

End results of simulations look familiar….

Dark Matter Dominates the Growth of Structure More dark matter than normal matter, so its gravitational pull is more significant Important characteristic of dark matter Speed of dark matter particles Hot vs. warm vs. cold Hot=speed close to speed of light Can determine this property without identifying the dark matter particle

Large vs. Small Structures Hot Dark MatterCold Dark Matter Time 1 Time 2 Time 1 Time 2

Hierarchical (bottom-up) formation

Top-down formation

Impact of Speed of DM particles Cold Dark Matter shows smaller collapsed structures than Warm Dark Matter (middle) or Hot Dark Matter (bottom) Galaxies grow larger by merging in a Cold Dark Matter Universe Much more “substructure” in a Cold Dark Matter Universe

Differences in Galaxy Formation

Evidence for Cold Dark Matter We predict that if the dark matter is cold: Galaxies start small & grow bigger by merging Lots of smaller galaxies in the Universe What kind of Universe do we live in? Lumpy! Evidence: Dwarf galaxies are common Smaller galaxies merging at high redshift Signs of past merging in the present-day MW

Galaxies from our time machine Elmegreen et al.

Movie of Milky Way formation Simulation with both dark matter and gas/stars Simulation shows the normal matter Gas can cool and collapse Rotation leads to disk formation Galaxy grows by accreting gas and small galaxies Mergers play an important role

Movie: ERIS simulation

Model of tidal tails Mergers leave behind tidal tails that are visible for ~billions of years after the main bodies have merged. Not always easy to find!

Milky Way swallowing the Sagittarius dwarf galaxy

Andromeda, our Sister Galaxy

Hey, it’s merging too!

The Sequel…..

The Nature of Dark Matter The story didn’t have to turn out this way. If dark matter is “hot”, then we would expect top-down formation. “Hot” dark matter moves very fast. Former dark matter candidate: the neutrino Neutrinos cannot make up the dark matter Neutrinos have very low masses “Cold” dark matter moves very slowly

Does theory predict too many small galaxies?

Open Questions Where are all the little galaxies? There should be oodles of little halos that haven’t merged yet But we don’t see oodles of galaxies Dark galaxies? Too much merging? Merging can destroy disks But we see lots of disks Gas reforms disks? Is it cold dark matter or warm dark matter?

Galaxies have a range of shapes