Luminosity‐dependent Quasar Lifetimes: Reconciling the Optical and X‐Ray Quasar Luminosity Functions

We consider implications of our new model of quasar lifetimes and lightcurves for the quasar luminosity function (LF) at different frequencies andredshifts. In our picture, quasars evolve rapidly and the lifetime depends onboth their instantaneous and peak luminosities. The bright end of the LF tracesthe peak intrinsic quasar activity, but the faint end consists of quasars whichare either undergoing exponential growth to much larger masses andluminosities, or are in sub-Eddington quiescent states going into or coming outof a period of peak activity. The 'break' in the observed LF correspondsdirectly to the maximum in the intrinsic distribution of peak luminosities,which falls off at both brighter and fainter luminosities. We study this modelusing simulations of galaxy mergers which successfully reproduce a wide rangeof observed quasar phenomena, including the observed column densitydistribution. By combining quasar lifetimes and the distribution of maximumquasar luminosities determined from the observed hard X-ray LF with thecorresponding luminosity and host-system dependent column densities, we producethe expected soft X-ray and B-band LFs. Our predictions agree exceptionallywell with the observed LFs at all observed luminosities, over the redshiftrange considered (z < 1), without invoking any ad hoc assumptions about anobscured population of sources. Our results also suggest that observedcorrelations in hard X-ray samples between the obscured fraction of quasars andluminosity can be explained in the context of our model by the expulsion ofsurrounding gas due to heating from accretion feedback energy as a quasar nearsits peak luminosity and final black hole mass.Comment: 9 pages, 4 figures. Accepted for publication in ApJ, October 2005. Revised and expanded with comments from referee repor