Pre-reionization Fossils, Ultra-faint Dwarfs and the Missing Galactic Satellite Problem

Understand

We argue that, at least a fraction of the newly discovered population of ultra-faint dwarf spheroidal galaxies in the Local Group constitute the fossil relic of a once ubiquitous population of dwarf galaxies formed before reionization with circular velocities smaller than $v_{c}^{cr} \sim 20$ km/s.

• We present several arguments in support of this model.
• The number of luminous Milky Way satellites inferred from observations is larger than the estimated number of dark halos in the Galaxy that have, or had in the past, circular velocity $>v_{c}^{cr}$, as predicted by the "Via Lactea" simulation.
• This implies that some ultra-faint dwarfs are fossils.