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New observations indicate that ultrafaint dwarf galaxies (UFD) -- the leastluminous systems bound by dark matter halos (<10^5 Lsun) -- may have formedbefore reionization. The extrapolated virial masses today are uncertain withestimates ranging from 10^8 Msun to 10^9 Msun. We show that the progenitor halomasses of UFDs can be as low as Mvir = 10^7 Msun. Under the right conditions,such a halo can survive the energy input of a supernova and its radiativeprogenitor. A clumpy medium is much less susceptible to both internal andexternal injections of energy. It is less prone to SN sweeping because thecoupling efficiency of the explosive energy is much lower than for a diffuseISM. With the aid of the 3D hydro/ionization code Fyris, we show thatsufficient baryons are retained to form stars following a single supernovaevent in dark matter halos down to Mvir ~ 10^7 Msun with radiative cooling. Thegas survives the SN explosion, is enriched with the abundance yields of thediscrete events, and reaches surface densities where low mass stars can form.Our highest resolution simulations reveal why cooling is so effective inretaining gas compared to any other factor. In the early stages, the super-hotmetal-enriched SN ejecta exhibit strong cooling, leading to much of theexplosive energy being lost. Consistent with earlier work, the baryons do *not*survive in smooth or adiabatic models in the event of a supernova. The smallestgalaxies carry signatures of the earliest epochs of star formation, which maydistinguish a small primordial galaxy from one that was stripped down to itspresent size through tidal interaction. We discuss these results in the contextof local UFDs and damped Ly-alpha systems (z~2) at very low metallicity ([Fe/H]~ -3). We show that both classes of objects are consistent with primordiallow-mass systems that have experienced only a few enrichment events.

Ultrafaint Dwarf Galaxies - the lowest mass relics from before reionization