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We argue that, at least a fraction of the newly discovered population ofultra-faint dwarf spheroidal galaxies in the Local Group constitute the fossilrelic of a once ubiquitous population of dwarf galaxies formed beforereionization with circular velocities smaller than $v_{c}^{cr} \sim 20$ km/s.We present several arguments in support of this model. The number of luminousMilky Way satellites inferred from observations is larger than the estimatednumber of dark halos in the Galaxy that have, or had in the past, circularvelocity $>v_{c}^{cr}$, as predicted by the "Via Lactea" simulation. Thisimplies that some ultra-faint dwarfs are fossils. However, this argument isweakened by recent results from the "Aquarius" simulations showing that thenumber of Galactic dark matter satellites is 2.5 larger than previouslybelieved. Secondly, the existence of a population of ultra-faint dwarfs waspredicted by cosmological simulations in which star formation in the firstminihalos is reduced -- but not suppressed -- by radiative feedback. Here, weshow the statistical properties of the fossil galaxies in those simulations areconsistent with observations of the new dwarf population and with the numberand radial distribution of Milky Way satellites as a function of theirluminosity. Finally, the observed Galactocentric distribution of dwarfs isconsistent with a fraction of dSphs being fossils. To make our case morecompelling, future work should determine whether stellar chemical abundances ofsimulated "fossils" can reproduce observations and whether the tidal scenariosfor the formation of Local Group dwarf spheroidals are equally consistent withall available observations.

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