On the formation of dwarf galaxies and stellar haloes

Using analytic arguments and a suite of very high resolution (10 3 M per particle) cosmological hydrodynamical simulations, we argue that high-redshift, z 10, M 10 8 M haloes, form the smallest 'baryonic building block' (BBB) for galaxy formation. These haloes are just massive enough to efficiently form stars through atomic line cooling and to hold on to their gas in the presence of supernova (SN) winds and reionization. These combined effects, in particular that of the SN feedback, create a sharp transition: over the mass range 3-10 10 7 M , the BBBs drop two orders of magnitude in stellar mass. Below 2 10 7 M , galaxies will be dark with almost no stars and no gas. Above this scale is the smallest unit of galaxy formation: the BBB.