Coevolution Between Supermassive Black Holes and Bulges Is Not Via Internal Feedback Regulation But By Rationed Gas Supply Due To Angular Momentum Distribution

TL;DR

This paper proposes that the coevolution of supermassive black holes and bulges is driven by the angular momentum distribution of infalling gas, rather than internal feedback mechanisms, supported by cosmological hydrodynamic simulations.

Contribution

It introduces a new scenario linking SMBH-bulge mass ratios to gas angular momentum distribution, tested with high-resolution simulations without AGN feedback.

Findings

SMBH to bulge mass ratio is 0.1-0.3% of the gas infall rate.

The ratio increases modestly with redshift, nearly redshift independent.

AGN duty cycle with high Eddington ratios increases with redshift.

Abstract

We reason that, without physical fine-tuning, neither the supermassive black holes (SMBHs) nor the stellar bulges can self-regulate or inter-regulate by driving away already fallen cold gas to produce the observed correlation between them. We suggest an alternative scenario where the observed mass ratios of the SMBHs to bulges reflect the angular momentum distribution of infallen gas such that the mass reaching the stable accretion disc is a small fraction of that reaching the bulge region, averaged over the cosmological time scales. We test this scenario using high resolution, large-scale cosmological hydrodynamic simulations (without AGN feedback), assuming the angular momentum distribution of gas landing in the bulge region to yield a Mestel disc that is supported by independent simulations resolving the Bondi radii of SMBHs. A mass ratio of $0.1-0.3\%$ between the very low angular momentum gas that free-falls to the sub-parsec region to accrete to the SMBH and the overall star formation rate is found. This ratio is found to increase with increasing redshift to within a factor of $\sim 2$, suggesting that the SMBH to bulge ratio is nearly redshift independent, with a modest increase with redshift, a testable prediction. Furthermore, the duty cycle of active galactic nuclei (AGN) with high Eddington ratios is expected to increase significantly with redshift. Finally, while SMBHs and bulges are found to coevolve on $\sim 30-150$Myr time scales or longer, there is indication that, on shorer time scales, the SMBH accretion rate and star formation may be less correlated.