COSMOLOGICAL EVOLUTION OF QUASARS

We present a model for the cosmological evolution of quasars (QSOs) un der the assumption that they are powered by massive accreting black ho les. Accretion flows around massive black holes make a transition from high radiative efficiency (similar to 10%) to low-efficiency, advecti on-dominated flows when M/M(Edd) falls below the critical rate similar to 0.3 alpha(2) similar to 10(-2) where ni is the mass accretion rate , M(Edd) proportional to M is the usual Eddington rate with the nomina l 10% efficiency, and alpha(less than or equal to 1) is the dimensionl ess viscosity parameter. We identify this transition with the observed break at a redshift similar to 2 in the QSO X-ray luminosity evolutio n. Growth of black holes through accretion could naturally lead to suc h a transition at a critical redshift z(c) similar to 1-3, provided th at most of the high-redshift QSOs appear with near-Eddington luminosit ies at z similar to 3-4 and the accretion rates decline over the Hubbl e time in a roughly synchronous manner. Before the transition, the QSO luminosities (with high efficiency) slowly decrease, and after the tr ansition at z(c) the QSO luminosities evolve approximately as (1 + z)( K(z)) ,where K(z) gradually varies from z = z(c) to z similar to 0 aro und K similar to 3. The results depend on the details of the QSO X-ray emission mechanism. We discuss some further implications.