Abstract

Recent work on the quasar luminosity function at optical and X-ray wavelengths is reviewed. It is shown that the evolution of the quasar luminosity function in these regimes is marked by a strong and approximately similar power law increase in luminosity, L ∝ (1 + z)3±0.5, between the present epoch and z ∼ 2. At z > 2, a slow-down in the rate of quasar evolution is witnessed in both regimes with possible evidence for a decrease in the space density of quasars being seen amongst optically faint (MB > −27) QSOs at z > 3.5.

Introduction

The quasar luminosity function (LF) is one of the most fundamental statistics relating to the quasar population. Estimates of the quasar LF and its evolution with redshift are normally obtained from the statistical analysis of large unbiased quasar surveys with complete spectroscopic identification. As such, the rapid increase in the number of such surveys in recent years, particularly in the optical and X-ray regimes, has led to a dramatic improvement in our knowledge of the quasar LF and its evolution. The purpose of this review is to describe the current observational status of the quasar LF in the optical (4400Å) and X-ray (∼ 2keV ≡ 6.2Å) regimes.

The Optical Luminosity Function

As a result of the recent improvement in quasar statistics at B > 20, it has become increasingly clear (Koo 1983, Marshall 1987, Boyle et al. 1988, Koo & Kron 1988) that the low redshift (z < 3) quasar optical LF (OLF) exhibits a significant break in its power law slope at faint absolute magnitudes.