High-redshift quasars in the Cold Dark Matter cosmogony

Abstract

We investigate the relationship between high-redshift quasars and the epoch of galaxy formation in the Cold Dark Matter (CDM) cosmogony. Luminous quasars could only form after galactic sized systems had collapsed. A constant comoving density of luminous quasars between z = 2 and z = 4 is compatible with the CDM model if quasars are short-lived and radiate at about the Eddington limit. However, according to the CDM model the abundance of high-luminosity quasars must decline exponentially at higher redshifts. Even if all protogalaxies form quasars, and about 1 per cent of the baryons within a protogalaxy collapse into a compact object, we would expect a steep fall in the density of quasars with L>10⁴⁷ erg s⁻¹ at redshifts z≳5. The existence of a ‘cut-off’ in the quasar numbers at high redshift could therefore supply an important test of the CDM theory.