Late Gas accretion onto Primordial Minihalos: a Model for Leo T, Dark Galaxies and Extragalactic High-Velocity Clouds

In this letter we revisit the idea of reionization feedback on dwarf galaxy formation. We show that primordial minihalos with v_cir<20 km/s stop accreting gas after reionization, as it is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases below z=3, they have a late phase of gas accretion and possibly star formation. We expect that pre-reionization fossils that evolved on the outskirts of the Milky Way or in isolation show a bimodal star formation history with 12 Gyr old and <10 Gyr old population of stars. Leo T fits with this scenario. Another prediction of the model is the possible existence of a population of gas rich minihalos that never formed stars. More work is needed to understand whether a subset of compact high-velocity clouds can be identified as such objects or whether an undiscovered population exists in the voids between galaxies.