DERIVING THE QUASAR LUMINOSITY FUNCTION FROM ACCRETION-DISK INSTABILITIES

We have derived the quasar luminosity function assuming that the quasa r activity is driven by a thermal-viscous unstable accretion disk arou nd a supermassive black hole. The instabilities produce large amplitud e, long-term variability of a single source. We take a light curve of a single source and calculate the luminosity function from the fractio n of time it spends at each luminosity. Convolving this with an assume d mass distribution we were able to fit well the observed optical lumi nosity function of quasars at four redshifts. As a result we obtain th e evolution of the mass distribution between redshifts 2.5 and 0.5. Th e main conclusions are as follows: (1) the quasar long-term variabilit y due to the disk thermal-viscous instabilities provides a natural exp lanation for the observed quasar luminosity function; (2) the peak of the mass function evolves toward lower black hole masses at lower reds hifts by a factor similar to 10; (3) the number of high-mass sources d eclines rapidly, so low-mass sources become dominant at lower redshift ; (4) outbursts of activity appear as long as the matter is supplied t o the accretion disk; (5) since the time-averaged accretion rate is lo w, the remnant sources (or sources in the low activity phase) do not g row into very massive black holes; and (6) a continuous fuel supply at a relatively low accretion rate (similar to 0.01-0.1 (M) overdot (Edd )) for each single source is required over the lifetime of the entire quasar population.