ULTRAFAINT DWARF GALAXIES - THE LOWEST-MASS RELICS FROM BEFORE REIONIZATION

Abstract

New observations suggest that ultrafaint dwarf galaxies (UFDs) - the least luminous systems bound by dark matter halos (≲105 L⊙) - may have formed before reionization. The extrapolated virial masses today are uncertain, with estimates ranging from 108 M⊙ to as high as 109 M⊙ depending on the assumed form of the underlying potential. We show that the progenitor halo masses of UFDs can, in principle, be as low as M<inf>vir</inf> ≈ 107 M⊙. Under the right conditions, such a halo can survive the energy input of a supernova (SN) and its radiative progenitor. A clumpy (fractal) medium is much less susceptible to both internal and external injections of energy. It is less prone to SN sweeping (particularly if it is off-centered) because the coupling efficiency of the explosive energy is much lower than for a diffuse interstellar medium. With the aid of the 3D hydro/ionization code Fyris, we show that sufficient baryons are retained to form stars following a single SN event in dark matter halos down to M<inf>vir</inf> ≈ 107 M⊙ in the presence of radiative cooling. In these models, the gas survives the SN explosion, is enriched with the specific abundance yields of the discrete events, and reaches surface densities where low-mass stars can form. Our highest-resolution simulations reveal why cooling is so effective in retaining gas compared to any other factor. In the early stages, the super-hot metal-enriched SN ejecta exhibit strong cooling, leading to much of the explosive energy being lost. Consistent with earlier work, the baryons do not survive in smooth or adiabatic models in the event of an SN. The smallest galaxies may not contribute a large fraction of matter to the formation of galaxies, but they carry signatures of the earliest epochs of star formation, as we show. These signatures may allow us to distinguish a small primordial galaxy from one that was stripped down to its present size through tidal interaction. We discuss these results in the context of local UFDs and damped Lyα systems (z ∼ 2) at very low metallicity ([Fe/H] ∼ -3). We show that both classes of objects are consistent with primordial low-mass systems that have experienced only a few enrichment events.