The History of Star Formation and the Colors of Late-Type Galaxies

Abstract

We have calculated the U, B, V colors of simple model galaxies which are characterized by three parameters: a, which specifies the mass distribution of stars at birth, p, the reciprocal decay time of an assumed exponential decay in the rate of star formation, and their age. The computations are based on theoretical evolutionary tracks for individual stars. The models for which p are shown to give a satisfactory account of the observed integrated colors of open clusters as a function of age. The observed colors of Sc and Irr I galaxies are found to be consistent with the following ideas. a) All galaxies are approximately 10" years old. b) An initial luminosity function qualitatively similar to the Salpeter function obtains in all late-type galaxies, but irregulars are somewhat more enriched in massive stars than Sc's are. c) The mean rate of star formation, averaged over sufficiently large regions of a galaxy and over sufficiently long times, is a declining function of time. d) Different galaxies of the same morphological type have very different decay times for star formation, and many galaxies have experienced roughly uniform rates of star formation for the last 10" years. We propose that the range in colors of the late-type galaxies reflects this range in decay times and point out that, if so, the frequency of Type II supernovae will depend on both the mass and the color of the parent galaxy. Finally we discuss the question whether the bluest known galaxies, all of which are dwarfs, are young. We tentatively conclude that, instead, they are galaxies which undergo intermittent and unusually intense bursts of star formation. Subject headings:galaxies - galaxies, photometry of - star formation - stellar statistics - supernovae