The Cosmological Evolution of Active Galaxies

Abstract

Our current knowledge of the cosmological evolution of radio galaxies, radio loud quasars, optically selected QSOs, X-ray selected QSOs and starburst galaxies is reviewed and summarised. Between z = 0 and z = 2 the level of evolution of these active galaxies is now reasonably well established; the luminosity functions of all five classes of object appear to display remarkably similar power-law luminosity evolution of the form L(z) ∞ (1 + z)³. However, the exact nature of this evolution remains uncertain and, in particular, the results of several recent surveys are now casting some doubt on the simple, ‘established’ picture of pure luminosity evolution for the QSO optical luminosity function. At z > 2 it is clear that this strong evolution in the luminosity functions of radio galaxies, radio quasars and optical QSOs must slow down and effectively cease. However, the evidence for a high-redshift reduction in their comoving density remains tantalising. For optically selected QSOs there is some evidence for a redshift cutoff at ‘intermediate’ luminosities (-27 < M _B < -26), but the space density of very luminous QSOs with M _B < -27 seems to remain unchanged between z = 2 and z = 4. The radio population may display qualitatively similar behaviour; there is evidence for a high-redshift ‘cutoff in the number of radio galaxies and quasars with P _2.7GHz≃ 10²⁶ WHz^-1sr^-1, but somewhat less clear evidence for any decline in the space density of sources with P _2.7GHz≃ 10²⁷ WHz^-1sr^-1. The significance of these results, and the prospects for improving their robustness are briefly discussed.