Quasars Are Not Light-Bulbs: Testing Models of Quasar Lifetimes with the Observed Eddington Ratio Distribution

摘要

We use the observed distribution of Eddington ratios as a function ofsupermassive black hole (BH) mass to constrain models of AGN lifetimes andlightcurves. Given the observed AGN luminosity function, a model for AGNlifetimes (time above a given luminosity) translates directly to a predictedEddington ratio distribution. Models for self-regulated BH growth, in whichfeedback produces a 'blowout' decay phase after some peak luminosity (shuttingdown accretion) make specific predictions for the lifetimes distinct from thoseexpected if AGN are simply gas starved (without feedback) and very differentfrom simple phenomenological 'light bulb' models. Present observations of theEddington ratio distribution, spanning 5 decades in Eddington ratio, 3 in BHmass, and redshifts z=0-1, agree with the predictions of self-regulated models,and rule out 'light-bulb', pure exponential, and gas starvation models at highsignificance. We compare the Eddington ratio distributions at fixed BH mass andfixed luminosity (both are consistent, but the latter are much lessconstraining). We present empirical fits to the lifetime distribution and showhow the Eddington ratio distributions place tight limits on AGN lifetimes atvarious luminosities. We use this to constrain the shape of the typical AGNlightcurve, and provide simple analytic fits. Given independent constraints onepisodic lifetimes, most local BHs must have gained their mass in no more thana couple of bright episodes, in agreement with merger-driven fueling models.