A Unified, Merger-Driven Model for the Origin of Starbursts, Quasars, the Cosmic X-Ray Background, Supermassive Black Holes and Galaxy Spheroids

Abstract

(Abridged) We present an evolutionary model for starbursts, quasars, and spheroidal galaxies in which mergers between gas-rich galaxies drive nuclear inflows of gas, producing intense starbursts and feeding the buried growth of supermassive black holes (BHs) until feedback expels gas and renders a briefly visible optical quasar. The quasar lifetime and obscuring column density depend on both the instantaneous and peak luminosity of the quasar, and we determine this dependence using a large set of simulations of galaxy mergers varying host galaxy properties, orbital geometry, and gas physics. We use these fits to deconvolve observed quasar luminosity functions (LFs) and obtain the evolution of the formation rate of quasars with a certain peak luminosity, n(L_peak,z). Quasars spend extended periods of time at luminosities well below peak, and so n(L_peak) has a maximum corresponding to the 'break' in the observed LF, falling off at both brighter and fainter luminosities. We obtain self-consistent fits to hard and soft X-ray and optical quasar LFs for a model in which n(L_peak) varies with redshift according to pure peak luminosity evolution. From n(L_peak) and our simulation results, we predict many observables, including: the column density distribution of both optical and X-ray samples, the LF of broad-line quasars in X-ray samples and the broad-line fraction as a function of luminosity, the low-z mass function of active BHs, the distribution of Eddington ratios at z~0-2, the z=0 mass function of relic BHs and total mass density of BHs, and the cosmic X-ray background. In every case, our predictions agree well with observed estimates, and unlike previous modeling attempts, we are able to reproduce them without invoking any ad hoc assumptions about source properties or distributions.