Physics Colloquium

The Growth of Supermassive Black Holes and Their Possible Effect on Galaxy Evolution

Using a “wedding cake” combination of multi-wavelength X-ray+infrared+optical surveys, we measure the growth of supermassive black holes at the centers of galaxies over the last ~12 billion years. Most actively growing black holes (“Active Galactic Nuclei” or AGN) are heavily obscured and thus look like inactive galaxies in optical surveys like the Sloan Digital Sky Survey (SDSS), so our census has effectively quadrupled the amount of accretion, and thus the amount of energy deposited in the host galaxies. Theorists have suggested that this energy could quench star formation and strongly affect galaxy evolution (“feedback”), and that AGN episodes are triggered by major galaxy mergers. Such a picture may hold for the most luminous quasars but our morphological analyses show that, both in the local universe and 7-9 billion years ago (at the peak of star formation and black hole growth), most galaxies are disk-dominated (i.e., not merger products) and they evolve too slowly for AGN to play a significant role. That is, there are two distinct modes of galaxy evolution, with mergers and AGN feedback affecting only a minority.