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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.