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