The cosmic evolution of galaxies and active galaxies

Evidence for strong evolutionary changes of the populations of extragalactic objects with cosmic epoch was first found in surveys of extragalactic radio sources and quasars in the 1950s and 1960s. An excess of faint sources was discovered in radio source and quasar surveys as compared with the expectations of uniform world models. The inference was that these classes of object were much more common at earlier cosmic epochs than they are at the present time. During the 1980s, the first deep counts of galaxies to very faint magnitudes became available thanks to the CCD revolution in optical detector technology. An excess of faint blue galaxies was discovered and these studies were extended to extremely faint apparent magnitudes by Hubble Space Telescope observations of the Hubble Deep Field and the Hubble Ultra-Deep Field.

This pattern of the discovery of excess numbers of faint objects at early cosmic epochs has been repeated in essentially all wavebands as deep surveys have become feasible. In the 1990s, surveys of the X-ray sky carried out by the ROSAT X-ray Observatory provided evidence for an excess of faint X-ray sources, similar to that found for the extragalactic radio sources and quasars. These studies were extended to much fainter X-ray sources by observations with the Chandra and XMM-Newton X-ray Observatories. The IRAS survey of the mid-and far-infrared sky, although not extending to as large redshifts as the radio and X-ray surveys, found evidence for an excess of faint sources.