I review recent results on the cosmological evolution of QSOs identified at optical, X-ray and radio frequencies. In all these regimes, it is now clear that the redshift range 2 ≲ z ≲ 3 corresponds to the epoch of maximum QSO activity. I demonstrate that QSO models invoking supermassive black holes or the starburst cores of young elliptical galaxies are equally successful at reproducing the observed space densities of even the most luminous QSOs in the Universe at these redshifts. In addition, both models can also account for the strong decline in QSO luminosity, L(z) ∝ (1 + z)3.0±0.4, observed in all regimes at lower redshifts (z ≲ 2). In the infrared and X-ray passbands, recent results suggest that starburst galaxies may also exhibit a remarkably similar rate of evolution to QSOs, L(z) ∝ (1 + z)3±1.

Introduction

In the last few years there has been a dramatic increase in the numbers of QSOs identified in complete spectroscopic surveys. In particular, a significant number of high redshift QSOs have now been identified, including more than 40 QSOs at z > 4. The mere existence of these high redshift QSOs, coupled with their inferred high metal abundances (Hamann & Ferland 1993; Ferland, this conference), indicates the presence of a significant number of massive, evolved systems only ∼ 1 Gyr after the Big Bang.