Abstract
Using a simple description of feedback from black hole growth, we develop an analytic model for the fueling of Seyferts (low-luminosity active galactic nuclei [AGNs]) and their relation to their host galaxies, Eddington ratio distributions, and cosmological evolution. We derive a solution for the time evolution of accretion rates in a feedback-driven blast wave, applicable to large-scale outflows from bright quasars in galaxy mergers, low-luminosity AGNs, and black holes or neutron stars in supernova remnants. Under the assumption that cold gas stochastically accretes onto a central supermassive black hole at a rate set by the dynamics of that gas, our solution determines the evolution of Seyfert light curves. Using this model, we predict the Seyfert luminosity function, duty cycles and AGN lifetimes, and the distribution of host morphologies, Eddington ratios, and obscuration as a function of AGN luminosity and black hole mass, and we find agreement with observations at z = 0. We consider the breakdown of the contribution from this mechanism and from stellar wind and virialized hot gas accretion and merger-driven activity. We also make specific predictions for the weak evolution of the Seyfert luminosity function; i.e., luminosity function of quiescent as opposed to merger-driven activity, as a function of redshift, and for changes in both the slope and scatter of the MBH-σ relation at low MBH. Our modeling provides a quantitative and physical distinction between local, low-luminosity quiescent AGN activity and violent, merger-driven bright quasars. In our picture, the quiescent mode of fueling dominates over a wide range of luminosities (-15 MB -22) at z = 0, where most black hole growth occurs in objects with MBH 107 M☉, in S0 and Sa/b galaxies. However, quasar activity from gas-rich mergers evolves more rapidly with redshift, and by z = 1, quiescent fueling is important only at luminosities an order of magnitude or more below the break in the luminosity function. Consequently, although non-merger-driven fueling is important for black hole growth and the MBH-σ relation at low MBH, it does not significantly contribute to the black hole mass density of the universe or to cosmological backgrounds.
Highlights
While it is generally accepted that quasars and active galactic nuclei (AGN) are powered by the accretion of gas onto supermassive black holes in the centers of galaxies (e.g. Salpeter 1964; Zel’dovich & Novikov 1964; Lynden-Bell 1969), the mechanism that fuels these objects is uncertain
Because the fueling mechanism is not cosmological in nature, these predictions are essentially a priori, and do not require a detailed cosmological modeling of the evolution in galaxy properties. This provides a context for considering such objects and their large-scale fueling mechanisms, and for contrasting them and their evolution with that of bright quasars driven by the cosmological processes of interactions and galaxy mergers
Individual bursts of accretion may have shorter timescales than the time-averaged light curves we calculate, our characteristic timescales are consistent with both the integrated and “episodic” quasar lifetime constraints from a number of observations, which estimate lower limits to the episodic lifetime of 104 yr, and direct attempts to measure the rates of short-duration X-ray flares have found rates ∼ 2 − 3 orders of magnitude below those needed to account for the low-luminosity AGN luminosity function (e.g., Donley et al 2002)
Summary
While it is generally accepted that quasars and active galactic nuclei (AGN) are powered by the accretion of gas onto supermassive black holes in the centers of galaxies (e.g. Salpeter 1964; Zel’dovich & Novikov 1964; Lynden-Bell 1969), the mechanism that fuels these objects is uncertain. The local AGN population largely comprises black holes in non-interacting, star-forming S0 and Sa/b hosts with no evidence of galaxy-scale perturbations or disturbances (Dong & De Robertis 2005), spanning most of the observed z = 0 AGN luminosity function (Hao et al 2005) These AGN have been more thoroughly studied and are more well-understood than bright quasars at high redshift, there is no self-consistent model for their triggering, fueling, and evolution. In a “collision,” the black hole will accrete at a high rate for a brief period of time until feedback impacts the cold gas, driving a blast wave and initiating a feedback-dominated “blowout” phase This blowout determines the subsequent, time-averaged evolution of the Seyfert light curve, obscured fractions, and Eddington ratio distributions, and the system decays to lower luminosities until a potential subsequent excitation.
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