Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback. II. Growth Rate and Duty Cycle

Abstract

This paper, the second of a series on radiation-regulated accretion onto black holes (BHs) from galactic scales, focuses on the effects of radiation pressure and angular momentum of the accreting gas. We simulate accretion onto intermediate-mass black holes, but we derive general scaling relationships that are solutions of the Bondi problem with radiation feedback valid for any mass of the BH $M_{\rm bh}$. Thermal pressure of the ionized sphere around the BH regulates the accretion rate producing periodic and short-lived luminosity bursts. We find that for ambient gas densities exceeding $n^{\rm cr}_{\rm H,\infty} \propto M_{\rm bh}^{-1}$, the period of the oscillations decreases rapidly and the duty cycle increases from 6%, in agreement with observations of the fraction of active galactic nuclei at $z\sim 3$, to 50%. The mean accretion rate becomes Eddington limited for $n_{\rm H,\infty}>n_{\rm H,\infty}^{\rm Edd} \simeq n_{\rm H,\infty}^{\rm cr} T_{\infty,4}^{-1}$ where $T_{\infty,4}$ is the gas temperature in units of $10^4$ K. In the sub-Eddington regime, the mean accretion rate onto BHs is about $1% T_{\infty,4}^{2.5}$ of the Bondi rate, thus is proportional to the thermal pressure of the ambient medium. The period of the oscillations coincides with depletion timescale of the gas inside the ionized bubble surrounding the BH. Gas depletion is dominated by a pressure gradient pushing the gas outward if $n_{\rm H,\infty}<n^{\rm cr}_{\rm H,\infty}$ and by accretion onto the BH otherwise. Generally, for $n_{\rm H,\infty}<n^{\rm cr}_{\rm H,\infty}$ angular momentum does not affect significantly the accretion rate and period of the oscillations.