Testing Models of Supermassive Black Hole Evolution with the Quasar Luminosity Function

The quasar luminosity function (QLF) describes the cosmic abundance of quasars at different luminosities and is a powerful tool in understanding how supermassive black holes (SMBHs) grow and evolve with their host galaxies. We compare the observed high-redshift QLF with predictions from four cosmological "zoom-in" simulations of massive galaxies from the FIRE project. The simulations predict a "kinked" relation between galaxy stellar mass and SMBH mass with three distinct regimes: (1) an early, slow growth phase regulated by stellar feedback, (2) a short phase of rapid growth, and (3) a later evolution along the local scaling relation. This prediction is in contrast to the commonly assumed linear relation between galaxy bulge mass and SMBH mass. We determine the Eddington ratio distributions associated with the three regimes, which we convolve with the stellar mass - halo mass relation and halo mass function to predict the QLF. The predicted QLF is consistent with the observed evolution of the QLF at high redshift (z ≥ 2). This indicates that, as predicted by the FIRE simulations, high-redshift SMBHs may grow efficiently in a relatively short period, following an earlier phase during which SMBH growth was suppressed by stellar feedback. In on-going work, we are quantifying more systematically the range of SMBH evolution models consistent with the observed QLF.