How low does it go? Too few Galactic satellites with standard reionization quenching

Understand

A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than Vpeak ~ 20 km/s, rendering the majority of haloes below this scale completely dark.

• We use a suite of cosmological zoom simulations of Milky Way-like haloes that include central Milky Way disk galaxy potentials to investigate the relationship between subhaloes and ultrafaint galaxies.
• We find that there are far too few subhaloes within 50 kpc of the Milky Way that had Vpeak > 20 km/s to account for the number of ultrafaint galaxies already known within that volume today.
• In order to match the observed count, we must populate subhaloes down to Vpeak ~ 6 km/s with ultrafaint dwarfs.