Growth of massive black holes (MBHs) in galactic centers comes mainly fromgas accretion during their QSO/AGN phases. In this paper we apply an extendedSoltan argument, connecting the local MBH mass function with the time-integralof the QSO luminosity function, to the demography of MBHs and QSOs from recentoptical and X-ray surveys, and obtain robust constraints on the luminosityevolution (or mass growth history) of individual QSOs (or MBHs). We find thatthe luminosity evolution probably involves two phases: an initial exponentiallyincreasing phase set by the Eddington limit and a following phase in which theluminosity declines with time as a power law (with a slope of -1.2--1.3) set bya self-similar long-term evolution of disk accretion. Neither an evolutioninvolving only the increasing phase with a single Eddington ratio nor anexponentially declining pattern in the second phase is likely. The period of aQSO radiating at a luminosity higher than 10% of its peak value is about(2-3)x10^8 yr, during which the MBH obtains ~80% of its mass. Themass-to-energy conversion efficiency is $0.16\pm0.04 ^{+0.05}_{-0}$, with thelatter error accounting for the maximum uncertainty due to Compton-thick AGNs.The expected Eddington ratios in QSOs from the constrained luminosity evolutioncluster around a single value close to 0.5-1 for high-luminosity QSOs andextend to a wide range of lower values for low-luminosity ones. The Eddingtonratios for high luminosity QSOs appear to conflict with those estimated fromobservations (~0.25) by using some virial mass estimators for MBHs in QSOsunless the estimators systematically over-estimate MBH masses by a factor of2-4. We also infer the fraction of optically obscured QSOs ~60-80%. Furtherapplications of the luminosity evolution of individual QSOs are also discussed.
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