Active Galactic Nucleus - Cosmological Uses and Evolution

Cosmological Uses and Evolution

For a long time, active galaxies held all the records for the highest-redshift objects known, because of their high luminosity (either in the optical or the radio): they still have a role to play in studies of the early universe, but it is now recognised that by its nature an AGN gives a highly biased picture of the 'typical' high-redshift galaxy.

More interesting is the study of the evolution of the AGN population. Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests (1) that massive black holes formed early on and (2) that the conditions for the formation of luminous AGN were more readily available in the early universe — for example, that there was a much higher availability of cold gas near the centre of galaxies than there is now. It also implies, of course, that many objects that were once luminous quasars are now much less luminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well constrained because of the difficulty of detecting and observing these objects at high redshifts.