Where are the Fossils of the First Galaxies? II. True Fossils, Ghost Halos, and the Missing Bright Satellites

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 within, and around the Milky Way. Throughout, we build upon previous results showing agreement between the observed stellar properties of a subset of the ultra-faint dwarfs and our simulated fossils. Here, we show that fossils of the first galaxies have galactocentric distributions and cumulative luminosity functions consistent with observations. In our model there are ~ 300 luminous satellites orbiting the Milky Way, ~50-70% of which are well preserved fossils, with this fraction decreasing with galactocentric distance. Within the Milky Way virial radius, the majority of these fossils have luminosities L_V<10^5 L_solar. This work produces an overabundance of bright dwarf satellites (L_V > 10^4 L_solar) with respect to observations where observations are nearly complete. The "bright satellite problem" is most evident in the outer parts of the Milky Way. We estimate that, although relatively bright, the primordial stellar populations are very diffuse, producing a population with surface brightnesses below surveys` detection limits and are easily stripped by tidal forces. Although we cannot yet present unmistakable evidence for the existence of the fossils of first galaxies in the Local Group, the results of our studies suggest observational strategies that may demonstrate their existence. Primarily, the detection of "ghost halos" of primordial stars around isolated dwarfs would prove that stars formed in minihalos (M<10^8 M_solar) before reionization, and strongly suggest that at least a fraction of the ultra-faint dwarfs are fossils of the first galaxies.