The observable universe contains about 100 billion galaxies. They are spirals and elliptical and irregulars and they live in groups and clusters and super clusters. Their rates of star formation depend on how much gas is recycled in the late stages of stellar evolution and how much new gas is available to fall in from the space between galaxies. Galaxies have been observed from the nearby universe to the dawn of time, half a billion years after the big bang. Is it possible to put all of this information together to tell the story of the star formation history of the universe?
The answer is a cautious yes. Large surveys have measured star formation across cosmic time and across all Hubble types. The universe is a quiet place compared to what it once was. As galaxies were being assembled by merger from smaller pieces, the rate of star formation was ferocious. In the smaller and denser universe there was abundant gas to fuel star formation. Overall, the rate of star formation peaked about 3.5 billion years after the big bang, at a red shift z = 2, and it has been declining exponentially since then. Half of the stellar mass seen today was formed over 8 billion year ago. The star formation rate now is 1/30 of its historic high. Star formation is sputtering out and most galaxies are old and dying. We're lucky to be living in one of the spirals that still has some vitality and youth.
This story is mirrored by the story of black hole growth at the centers of galaxies. The super massive black holes were assembled early and experienced their peak of activity at z = 2, when the space density of quasar activity was hundreds of time higher than it is today. Starved of fuel, most of these central black holes have become silent. Another trend that has been seen in galaxy surveys is called downsizing. The hierarchical view of structure formation is that small galaxies formed first and merged to produce larger galaxies over cosmic time. If star formation tracks the typical size of most galaxies then most early star formation should have been in small galaxies, with later star formation in large galaxies like the Milky Way. In fact, what is seen is almost the opposite — most early star formation in the universe seems to have occurred in large galaxies, but as the universe aged that star formation stopped, even as it continued in smaller galaxies. Something analogous has been seen in active galaxies too. In recent times, the peak of quasar space density occurs for modest black holes, not the most massive ones. Downsizing is still a very active area for research but the mechanisms that control star formation and black hole growth are clearly more complex than astronomers once thought. It may take future large survey to gain a better understanding.