Title: Reionization Fossils, Ghost Halos and the Missing Bright Satellites

Mia S. Bovill and Massimo Ricotti

Abstract:

We use a new set of cold dark matter simulations of the local universe to investigate the distribution of fossils of primordial 
dwarf galaxies around the Milky Way. We build upon previous results showing agreement between observed stellar properties 
of a subset of the ultra-faint dwarfs and our simulated fossils. However, we find an overabundance of bright (L_V > 10^4 
L_solar) satellites with respect to observations where they are nearly complete. This “bright satellite problem’ is most evident 
in the outer parts of the Milky Way, but is present at radii as small as 50 kpc. We estimate the bright satellites’ primordial 
stellar populations may be extremely diffuse, producing ghost halos with surface brightnesses below surveys‘ detection limits 
and easily stripped by tidal forces. Although we cannot yet present unmistakable evidence for the existence of the primordial 
fossils in the Local Group, the detection of “ghost halos’ of primordial stars around isolated dwarfs would prove that stars 
formed in minihalos before reionization, and strongly suggest that a fraction of the ultra-faint dwarfs are fossils of the first 
galaxies.