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We apply a simple, one-equation, galaxy formation model on top of the halosand subhalos of a high-resolution dark matter cosmological simulation to studyhow dwarf galaxies acquire their mass and, for better mass resolution, on over10^5 halo merger trees, to predict when they form their stars. With the firstapproach, we show that the large majority of galaxies within group- andcluster-mass halos have acquired the bulk of their stellar mass through gasaccretion and not via galaxy mergers. We deduce that most dwarf ellipticals arenot built up by galaxy mergers. With the second approach, we constrain the starformation histories of dwarfs by requiring that star formation must occurwithin halos of a minimum circular velocity set by the evolution of thetemperature of the IGM, starting before the epoch of reionization. Wequalitatively reproduce the downsizing trend of greater ages at greater massesand predict an upsizing trend of greater ages as one proceeds to masses lowerthan m_crit. We find that the fraction of galaxies with very young stellarpopulations (more than half the mass formed within the last 1.5 Gyr) is afunction of present-day mass in stars and cold gas, which peaks at 0.5% atm_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. Wepredict that the baryonic mass function of galaxies should not show a maximumat masses above 10^5.5, M_Sun, and we speculate on the nature of the lowestmass galaxies.

How do dwarf galaxies acquire their mass & when do they form their stars?