The build-up of massive galaxies

Abstract

Although we have a broad picture of how the universe has evolved from the Big Bang more than 13.7 billion years ago, the details of the formation and evolution of galaxies are still not well understood. Using a combination of optical and infrared observations called the SIMPLE survey, it is possible to picture galaxies at different stages in their evolution. It appears that the star formation rate per unit mass, the specific star formation rate, has been steadily decreasing for over the last 10 billion years. During this period it was always lower for the most massive galaxies but the rate of change does not seem to be a strong function of mass. Using the specific star formation rate, it is possible to determine the fraction of galaxies that are passively evolving. It appears that 30% of the most massive galaxies already formed the bulk of their stars 10 billion years ago and have been passively evolving at least since then. This measurement can be used as a constraint for models of galaxy evolution. A comparison between such a model and the SIMPLE observations, shows that the models can not predict the correct growth rate through star formation well enough yet.