Black Hole Growth in Disk Galaxies Mediated by the Secular Evolution of Short Bars

TL;DR

This paper investigates how short bars in disk galaxies influence black hole growth, finding that massive black holes can destroy these bars, which limits black hole mass growth via this secular process.

Contribution

It demonstrates that short bars can be destroyed by black holes exceeding 0.2% of stellar mass, setting a maximum black hole mass in non-merger galaxies.

Findings

Short bars are destroyed by black holes of 0.05-0.2% of stellar mass.

Observed galaxies with short bars have black holes below 0.2% of stellar mass.

Maximum black hole mass via inner bars is about 0.2% of galaxy stellar mass.

Abstract

The growth of black holes (BHs) in disk galaxies lacking classical bulges, which implies an absence of significant mergers, appears to be driven by secular processes. Short bars of sub-kiloparsec radius have been hypothesized to be an important mechanism for driving gas inflows to small scale, feeding central BHs. In order to quantify the maximum BH mass allowed by this mechanism, we examine the robustness of short bars to the dynamical influence of BHs. Large-scale bars are expected to be robust, long-lived structures; extremely massive BHs, which are rare, are needed to completely destroy such bars. However, we find that short bars, which are generally embedded in large-scale outer bars, can be destroyed quickly when BHs of mass $M_{\rm bh}\sim0.05-0.2\%$ of the total stellar mass ($M_\star$) are present. In agreement with this prediction, all galaxies observed to host short bars have BHs with a mass fraction less than $0.2\%M_\star$. Thus the dissolution of short inner bars is possible, perhaps even frequent, in the universe. An important implication of this result is that inner bar-driven gas inflows may be terminated when BHs grow to $\sim0.1\%M_\star$. We predict that $0.2\%M_\star$ is the maximum mass of BHs allowed if they are fed predominately via inner bars. This value matches well the maximum ratio of BH-to-host galaxy stellar mass observed in galaxies with pseudo-bulges and most nearby AGN host galaxies. This hypothesis provides a novel explanation for the lower $M_{\rm bh}/M_\star$ in galaxies which have avoided significant mergers compared with galaxies with classical bulges.