The IMACS Cluster Building Survey. III. The Star Formation Histories of
  Field Galaxies
  Using data from the IMACS Cluster Building Survey and from nearby galaxy
surveys, we examine the evolution of the rate of star formation in field
galaxies from $ z = 0.60$ to the present. Fitting the luminosity function to a
standard Schechter form, we find a rapid evolution of $M_B^*$ consistent with
that found in other deep surveys; at the present epoch $M_B^*$ is evolving at
the rate of $0.38 Gyr^{-1}$, several times faster than the predictions of
simple models for the evolution of old, coeval galaxies. The evolution of the
distribution of specific star formation rates (SSFR) is also too rapid to
explain by such models. We demonstrate that starbursts cannot, even in
principle, explain the evolution of the SSFR distribution. However, the rapid
evolution of both $M_B^*$ and the SSFR distribution can be explained if some
fraction of galaxies have star formation rates characterized by both short rise
and fall times and by an epoch of peak star formation more recent than the
majority of galaxies. Although galaxies of every stellar mass up to
$1.4\times10^{11}\Msun$ show a range of epochs of peak star formation, the
fraction of "younger" galaxies falls from about 40% at a mass of
$4\times10^{10}\Msun$ to zero at a mass of $1.4\times10^{11}\Msun$. The
incidence of younger galaxies appears to be insensitive to the density of the
local environment; but does depend on group membership: relatively isolated
galaxies are much more likely to be young than are group members.
