Predicting the frequencies of young and of tiny galaxies
  A simple, 1-equation, galaxy formation model is applied to both the halo
merger tree derived from a high-resolution dissipationless cosmological
simulation and to 1/4 million Monte-Carlo halo merger trees. The galaxy
formation model involves a sharp entropy barrier against the accretion of gas
onto low-mass halos, the shock heating of infalling gas far from the central
regions of massive halos, and supernova feedback that drives the gas out of
shallow halo potential wells. With the first approach, we show that the large
majority of galaxies within group- and cluster-mass halos, known to be mainly
dwarf ellipticals, have acquired the bulk of their stellar mass through gas
accretion and not via galaxy mergers. With the second approach, we
qualitatively reproduce the downsizing trend of greater ages at greater masses
in stars and predict an upsizing trend of greater ages as one proceeds to
masses lower than 10^10 M_Sun. We find that the fraction of galaxies with very
young stellar populations (more than half the stellar mass formed within the
last 1.5 Gyr) is a function of present-day stellar mass, which peaks at 0.5% at
m_crit=10^7.5-9.5 M_Sun, roughly corresponding to the masses of blue compact
dwarfs. We predict that the stellar mass function of galaxies should not show a
maximum at m_stars > 10^{5.5}, M_Sun, with a power-law stellar mass function
with slope approx -1.6 if the IGM temperature in the outskirts of halos before
reionization is set by H2 cooling. We speculate on the nature of the lowest
mass galaxies.
