How low does it go? Too few Galactic satellites with standard
  reionization quenching
  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. The required
haloes have peak virial temperatures as low as 1,500 K, well below the atomic
hydrogen cooling limit of 10^4 K. Allowing for the possibility that the Large
Magellanic Cloud contributes several of the satellites within 50 kpc could
potentially raise this threshold to 10 km/s (4,000 K), still below the atomic
cooling limit and far below the nominal reionization threshold.
