A Physical Model for the Origin of Quasar Lifetimes

We propose a model of quasar lifetimes in which observational quasar lifetimes and an intrinsic lifetime of rapid accretion are strongly distinguished by the physics of obscuration by surrounding gas and dust. Quasars are powered by gas funneled to galaxy centers, but for a large part of the accretion lifetime they are heavily obscured by the large gas densities powering accretion. During this obscured phase, starbursts and black hole growth are fueled but the quasar is buried. Eventually, feedback from the accretion energy disperses surrounding gas and creates a window in which the black hole is observable optically as a quasar, until the accretion rate drops below that required to maintain a quasar luminosity. We model this process and measure the unobscured and intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxy merger. The bolometric luminosity of the source is determined from the black hole accretion rate, calculated from the local gas properties. We calculate the column density of hydrogen to the central galactic black hole along multiple lines-of-sight in the simulation, and use these column densities and the gas metallicity to determine the B-band attenuation of the central source. Defining the observable quasar lifetime as the total time with an observed B-band luminosity greater than some lower limit LB,min, we find lifetimes ∼ 10− 20 Myr for LB,min = 1011 L⊙ (MB ≈ −23), in good agreement with observationally determined quasar lifetimes. These numbers are significantly smaller than the “intrinsic” lifetime ∼ 100 Myr obtained if attenuation is neglected. We find similar lifetimes defined by an observed bolometric luminosity greater than 10% of the Eddington luminosity. The ratio of observed lifetimes to intrinsic lifetime is also strong function of both the limiting luminosity and the observed frequency. Subject headings: quasars: general — galaxies: nuclei — galaxies: active — galaxies: evolution — cosmology: theory