The missing ultra-faint satellites of the Milky Way

Abstract

We combine the highest resolution N-body simulation of a ∼1012M⊙ ΛCDM halo (Aquarius-A) with the semi-analytic model GALFORM to study the full satellite population of Milky Way (MW)-like galaxies. The model assumes that galaxies form only in subhalos whose peak circular velocity exceeds the hydrogen-cooling threshold, and predicts that the total number of satellites ever accreted into the main halo, down to arbitrarily low luminosity, should not exceed a few hundred. The model tracks satellites even after their halos become unresolved (”orphan” galaxies), and includes a novel treatment of dark matter and stellar tidal stripping which takes into account that ΛCDM subhalos survive tidal disruption because of their cuspy density profiles. Our results match the massive end of the observed MW satellite mass function and predict that many ultra-faint dwarfs (UFDs) with MV > −5 are missing from the current MW satellite census. The missing UFDs are predicted to avoid the innermost regions of the host, and have properties overlapping those of the recently-discovered ultra-faint compact MW satellites (UFCSs). If the faintest dwarfs are initially as compact as UFCSs, our results suggest that many UFCSs should be dark matter-dominated dwarfs with velocity dispersions 1 − 3 km/s which survived disruption because they inhabit the cusp of ΛCDM subhalos. UFCSs have predicted densities of 1010-1011 M⊙/kpc3, higher than those of more extended UFDs. If confirmed, our results provide strong support for the cuspy nature of ΛCDM halos and for the hydrogen-cooling threshold for galaxy formation.