Late gas accretion on to primordial minihaloes: A model for Leo T, dark galaxies and extragalactic high-velocity clouds

Abstract

In this Letter, we revisit the idea of reionization feedback on dwarf galaxy formation. We show that primordial minihaloes with νcir < 20 km s-1 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- 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 minihaloes 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 of dark galaxies exists in the voids between galaxies. © 2008 The Author. Journal compilation © 2008 RAS.