2009MNRAS.395L...6S

We investigate the nature of the newly discovered Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context simultaneously accounting for various `classical' dSphs and Milky Way properties including their metallicity distribution function (MDF). To this aim, we extend the merger tree approach previously developed to include the presence of star-forming minihaloes, and a heuristic prescription for radiative feedback. The model successfully reproduces both the observed [Fe/H]-luminosity relation and the mean MDF of UFs. In this picture, UFs are the oldest, most dark matter-dominated (M/L > 100) dSphs with a total mass M = 10^7–8M_☉; they are leftovers of H₂-cooling minihaloes formed at z > 8.5, that is before reionization. Their MDF is broader (because of a more prolonged star formation) and shifted towards lower [Fe/H] (as a result of a lower gas metallicity at the time of formation) than that of classical dSphs. These systems are very ineffectively star-forming, turning into stars by z = 0 only <3 per cent of the potentially available baryons. We provide a useful fit for the star formation efficiency of dSphs.