cosmological concerns i: missing satellite problem bullock...

Cosmological Concerns I: Missing Satellite Problem

Dark Matter simulations of MW galaxies look like this: But we only see this:

Bullock & Boylan-Kolchin, ARAA, 2017

https://www.annualreviews.org/doi/full/10.1146/annurev-astro-091916-055313


Solving Missing Satellites: Need to say that galaxy forma7on becomes extremely inefficient at low dark halo mass. So lots of low mass halos exist, they just dont have galaxies in them.

Why would low mass dark maBer halos fail to make/host galaxies?

Shallow poten8al wells: Any early star forma7on may result in energe7c “feedback” (stellar winds, supernovae) that blow gas out of galaxy.

“Proximity Effect”: star forma7on in a nearby massive galaxy may do the same thing.

Reioniza8on: At higher redshiK, before stars can form, the gas has cooled from the Big Bang and can collapse into dark maBer halos. Once star forma7on kicks in (around ! ≈ 6 − 10), the young stars ionize gas throughout the universe, hea7ng it up so that it can’t collapse into low mass halos:

32 )*+,- >

/01,234




Solving Missing Satellites: Need to say that galaxy forma7on becomes extremely inefficient at low dark halo mass. So lots of low mass halos exist, they just dont have galaxies in them.

M*/Mvir ≈ 0.01

M*/Mvir ≈ 0.001

M*/Mvir ≈ 0.0001

If the stellar mass (M*) – dark halo mass (Mvir) rela7on looks like this:

Then you could get a mass func7on that looks like this:


Simulated Mass Functionof Dwarf Galaxies


Cosmological Concerns II: Cusp-Core

Dark matter models predict density profiles that are very dense at the center (cuspy), which should give rise to a very

steeply rising rotation curve. Instead, dwarf galaxies have slowly rising rotation curves which imply a constant density

core to the dark matter distribution.


Dark matter simulations: !"# ~ %&' in the center.

Observations say () ~ % in the inner regions. This would mean

!"# ~*(%)%- ~ %().

%- ~ %-%- ~ /0123

Solving Cusp-Core:

• “Self-interacting dark matter” models. Dark matter particles

interact with one another in a way that sets up a density

core on small scales.

• “Baryonic heating”: star formation and feedback in the

inner regions drives gas outwards, reduces central density,

dark matter expands outwards a bit as well. Forms a core.


Cosmological Concerns III: Too Big to Fail

The Milky Way satellites that do host galaxies are all lower in mass than the most massive subhalos predicted in dark ma;er models:


Purple lines: Vcirc for simulated subhalos.Black points: data for observed MW satellites.

These higher Vcirc halos in the simulations are too massive to have failed to form stars. So where are they?

Solving TBTF:

• Baryonic heating reduces central mass density, lowers Vcirc

• Other dynamical processes (tidal stripping, disk shocking, etc) reduce central mass density, lowers Vcirc

• Systematic uncertainties in measuring Vcirc for low mass galaxies.


cosmological concerns i: missing satellite problem bullock...

Documents