A Physical Model for the Origin of Quasar Lifetimes

We propose a model of quasar lifetimes in which observational quasarlifetimes and an intrinsic lifetime of rapid accretion are stronglydistinguished by the physics of obscuration by surrounding gas and dust.Quasars are powered by gas funneled to galaxy centers, but for a large part ofthe accretion lifetime are heavily obscured by the large gas densities poweringaccretion. In this phase, starbursts and black hole growth are fueled but thequasar is buried. Eventually, feedback from accretion energy dispersessurrounding gas, creating a window in which the black hole is observableoptically as a quasar, until accretion rates drop below those required tomaintain a quasar luminosity. We model this process and measure the unobscuredand intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxymerger. The source luminosity is determined from the black hole accretion rate,calculated from local gas properties. We calculate the column density ofhydrogen to the source along multiple lines of sight and use these columndensities and gas metallicities to determine B-band attenuation of the source.Defining the observable quasar lifetime as the total time with an observedB-band luminosity above some limit L_B,min, we find lifetimes ~10-20 Myr forL_B,min=10^11 L_sun (M_B=-23), in good agreement with observationallydetermined quasar lifetimes. This is significantly smaller than the intrinsiclifetime ~100 Myr obtained if attenuation is neglected. The ratio of observedto intrinsic lifetime is also strong function of both the limiting luminosityand the observed frequency.