The Evolution of Galaxies and Active Galaxies with Cosmic Epoch

Over recent years, there has been a deluge of data concerning changes with cosmic epoch of essentially all classes of extragalactic object. Evidence for strong evolutionary changes with cosmic epoch was first found in the 1950s and 1960s as a result of surveys of extragalactic radio sources and quasars. The key evidence was the excess of faint sources found in radio source and quasar surveys, as compared with the expectations of all uniform world models. The inference was that there were many more of these classes of objects at early cosmic epochs as compared with their number at the present epoch. During the 1980s, as the first deep counts of galaxies became available, it was discovered that there is a large excess of blue galaxies at faint apparent magnitudes. These studies culminated in the remarkable observations of the Hubble Deep Field by the Hubble Space Telescope, which will be discussed in some detail in Chap. 18.