10 billion years of massive Galaxies

Abstract

The most massive galaxies in the local universe are not forming new stars -- but we don__t know why. As a step towards figuring out why big galaxies stop forming stars, we set out to measure when they stop forming stars. By looking at the colors of massive galaxies have changed over 10 billion years of cosmic time (roughly 80% of the history of the universe), we found that around 1/6 massive galaxies had already finished their star formation when the universe was just a few billion years old, and around 1/2 were still forming stars about 7 billion years ago. But the story doesn__t stop there. Even after galaxies have stopped forming stars they somehow keep growing in size: the massive, quiescent galaxies that existed 10 billion years ago are 3-10 times more compact than the ones we see today. We confirmed this fact by showing that there really are no galaxies in the local universe as small as those that existed 10 billion years ago. A major feature of this work is detailed and systematic study of the importance of different kinds of errors in these results. In this way, our results are more robust than those that have come before.