Revisiting Soltan's argument based on a semi-analytical model for galaxy and black hole evolution

TL;DR

This paper demonstrates that super-Eddington accretion plays a significant role in the growth of supermassive black holes, challenging traditional interpretations of Soltan's argument and providing a semi-analytical model consistent with observations.

Contribution

It introduces a semi-analytical model showing super-Eddington accretion's importance in SMBH growth, revisiting Soltan's argument with new insights.

Findings

Super-Eddington accretion contributes significantly to SMBH mass growth.

SMBHs with > 10^9 Msun acquire over 50% of their mass via super-Eddington phases.

Mass-weighted radiation efficiency at z=0 is about 0.08, consistent with Soltan's argument.

Abstract

We show the significance of the super-Eddington accretion for the cosmic growth of supermassive black holes (SMBHs) with a semi-analytical model for galaxy and black hole evolution. The model explains various observed properties of galaxies and active galactic nuclei at a wide redshift range. By tracing the growth history of individual SMBHs, we find that the fraction of the SMBH mass acquired during the super-Eddington accretion phases to the total SMBH mass becomes larger for less massive black holes and at higher redshift. Even at z = 0, SMBHs with > 1e+9 Msun have acquired more than 50% of their mass by super-Eddington accretions, which is apparently inconsistent with classical Soltan's argument. However, the mass-weighted radiation efficiency of SMBHs with > 1e+8 Msun obtained with our model, is about 0.08 at z = 0, which is consistent with Soltan's argument within the observational uncertainties. We, therefore, conclude that Soltan's argument cannot reject the possibility that SMBHs are grown mainly by super-Eddington accretions.