  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. However, this argument is
weakened by recent results from the "Aquarius" simulations showing that the
number of Galactic dark matter satellites is 2.5 larger than previously
believed. Secondly, the existence of a population of ultra-faint dwarfs was
predicted by cosmological simulations in which star formation in the first
minihalos is reduced -- but not suppressed -- by radiative feedback. Here, we
show the statistical properties of the fossil galaxies in those simulations are
consistent with observations of the new dwarf population and with the number
and radial distribution of Milky Way satellites as a function of their
luminosity. Finally, the observed Galactocentric distribution of dwarfs is
consistent with a fraction of dSphs being fossils. To make our case more
compelling, future work should determine whether stellar chemical abundances of
simulated "fossils" can reproduce observations and whether the tidal scenarios
for the formation of Local Group dwarf spheroidals are equally consistent with
all available observations.
