Cosmological luminosity evolution of the QSO/active galactic nucleus population

Abstract

We apply the observed optical/X-ray spectral states of the Galactic black hole candidates (GBHCs) to the cosmological QSO luminosity evolution under the assumptions that QSOs and GBHCs are powered by similar accretion processes and that their emission mechanisms are also similar. The QSO luminosity function (LF) evolution in various energy bands is strongly affected by the spectral evolution, which is tightly correlated with the luminosity evolution. We generate a random sample of QSOs born nearly synchronously by allowing the QSOs to have redshifts in a narrow range around an initial high redshift, black hole masses according to a power law, and mass accretion rates near Eddington rates. The QSOs evolve as a single long-lived population on the cosmological timescale. The pure luminosity evolution results in distinct luminosity evolution features caused by the strong spectral evolution. Most notably, different energy bands (optical/UV, soft X-ray, and hard X-ray) show different evolutionary trends, and the hard X-ray LF in particular shows an apparent reversal of the luminosity evolution (from decreasing to increasing luminosity) at low redshifts, which is not seen in the conventional pure luminosity evolution scenario without spectral evolution. The resulting mass function of black holes (BHs), which is qualitatively consistent with the observed QSO LF evolution, shows that QSO remnants are likely to be found as BHs with masses in the range 108-5 x 1010 M⊙. The long-lived single population of QSOs are expected to leave their remnants as supermassive BHs residing in rare, giant elliptical galaxies.