Luminosity‐dependent Quasar Lifetimes: A New Interpretation of the Quasar Luminosity Function

We propose a new interpretation of the quasar luminosity function (LF),derived from physically motivated models of quasar lifetimes and light curves.In our picture, quasars evolve rapidly and their lifetime depends on both theirinstantaneous and peak luminosities. We study this model using simulations ofgalaxy mergers that successfully reproduce a wide range of observed quasarphenomena. With lifetimes inferred from the simulations, we deconvolve theobserved quasar LF from the distribution of peak luminosities, and show thatthey differ qualitatively, unlike for the simple models of quasar lifetimesused previously. We find that the bright end of the LF traces the intrinsicpeak quasar activity, but that the faint end consists of quasars which areeither undergoing exponential growth to much larger masses and higherluminosities, or are in sub-Eddington quiescent states going into or coming outof a period of peak activity. The 'break' in the LF corresponds directly to themaximum in the intrinsic distribution of peak luminosities, which falls off atboth brighter and fainter luminosities. Our interpretation of the quasar LFprovides a physical basis for the nature and slope of the faint-enddistribution, as well as the location of the break luminosity.