The Relation between Quasar and Merging Galaxy Luminosity Functions and the Merger-driven Star Formation History of the Universe

Using a model for the self-regulated growth of supermassive black holes in mergers involving gas-rich galaxies, we study the relationship between quasars and the population of merging galaxies and thereby predict the merger-driven star formation rate density of the Universe. In our picture, mergers drive gas inflows, fueling nuclear starbursts and “buried” quasars until feedback disperses the gas, allowing the quasar to be briefly visible as a bright optical source. As black hole accretion declines, the quasar dies and the stellar remnant relaxes passively with properties and correlations typical of red, elliptical galaxies. By simulating the evolution of such events, we demonstrate that the observed statistics of merger rates/fractions, luminosity functions, mass functions, star formation rate distributions, quasar luminosity functions, quasar host galaxy luminosity functions, and elliptical/red galaxy luminosity and mass functions are self-consistent. We use our simulations to de-convolve both the quasar and merging galaxy luminosity functions to determine the birthrate of black holes of a given final mass and merger rates as a function of the total stellar mass and the mass of new stars formed during a merger. From this, we predict the merging galaxy luminosity function in various observed wavebands (e.g. UV, optical, and near-IR), color-magnitude relations, mass functions, absolute and specific star formation rate distributions and star formation rate density, and quasar host galaxy luminosity function, as a function of redshift from z = 0 − 6. We invert this relationship to predict e.g. quasar luminosity functions from observed merger luminosity functions or star formation rate distributions. Our results show good agreement with observations, but idealized models of quasar lightcurves give inaccurate estimates and are ruled out by comparison of merging galaxy and quasar observations at > 99.9% confidence, provided that quasars are triggered in mergers. Using only observations of quasars, we estimate the contribution of mergers to the star formation rate density of the Universe to high redshifts, z ∼ 4, and constrain the evolution in the characteristic initial gas fractions of quasar and spheroid-producing mergers. Subject headings: quasars: general — galaxies: active — galaxies: evolution — cosmology: theory