The observed $M - \sigma$ relations imply that SMBHs grow by cold chaotic accretion

TL;DR

This paper uses observed $M - \sigma$ relations to argue that supermassive black holes grow primarily through chaotic accretion of gas clouds, rather than large-scale planar gas flows like discs or bars.

Contribution

It provides a theoretical and observational analysis showing that SMBHs are unlikely to be fed by large-scale planar gas flows, favoring chaotic accretion as the dominant growth mechanism.

Findings

SMBHs in pseudobulge galaxies are under-massive compared to classical bulges.

Planar gas flows are inefficient for SMBH growth due to star formation.

Chaotic accretion explains the observed $M - \sigma$ relations.

Abstract

We argue that current observations of $M - \sigma$ relations for galaxies can be used to constrain theories of super-massive black holes (SMBH) feeding. In particular, assuming that SMBH mass is limited only by the feedback on the gas that feeds it, we show that SMBHs fed via a planar galaxy scale gas flow, such as a disc or a bar, should be much more massive than their counterparts fed by quasi-spherical inflows. This follows from the relative inefficiency of AGN feedback on a flattened inflow. We find that even under the most optimistic conditions for SMBH feedback on flattened inflows, the mass at which the SMBH expels the gas disc and terminates its own growth is a factor of several higher than the one established for quasi-spherical inflows. Any beaming of feedback away from the disc and any disc self-shadowing strengthens this result further. Contrary to this theoretical expectation, recent observations have shown that SMBH in pseudobulge galaxies (which are associated with barred galaxies) are typically under- rather than over-massive when compared with their classical bulge counterparts at a fixed value of $\sigma$. We conclude from this that SMBHs are not fed by large (100 pc to many kpc) scale gas discs or bars, most likely because such planar flows are turned into stars too efficiently to allow any SMBH growth. Based on this and other related observational evidence, we argue that most SMBHs grow by chaotic accretion of gas clouds with a small and nearly randomly distributed direction of angular momentum.