The Origins of Gas Accreted by Supermassive Black Holes: the Importance of Recycled Gas

TL;DR

This study uses cosmological simulations to show that recycled gas from dying stars is the main fuel for supermassive black holes, with galaxy mergers also contributing but on longer timescales, complicating the AGN-merger connection.

Contribution

It identifies recycled stellar gas as the dominant fueling source for SMBHs and quantifies the timescales and roles of mergers in SMBH accretion.

Findings

Recycled stellar gas is the primary fuel for SMBHs.

Galaxy mergers contribute to SMBH fueling, especially in massive halos.

It takes about 1.85 Gyr for external gas to reach and fuel the SMBH.

Abstract

We investigate the fueling mechanisms of supermassive black holes (SMBHs) by analyzing ten zoom-in cosmological simulations of massive galaxies, with stellar masses $10^{11-12} M_{\odot}$ and SMBH masses $10^{8.9-9.7}$ at $z=0$ and featuring various major and minor merger events. By tracing the gas history in these simulations, we categorize the gas accreted by the central SMBHs based on its origin. Gas that belonged to a different galaxy before accretion onto the BH is labeled as (1) ``external," while smoothly accreted cosmic gas is classified as (2) ``smooth." Gas produced within the primary halo through stellar evolution and subsequently accreted by the SMBH is classified as (3) ``recycled." Our analysis, which included stellar feedback, reveals that the primary fuel source for SMBHs is the recycled gas from dying stars. This recycled gas from stars in the inner region of the galaxy readily collapses toward the center, triggering starbursts, and simultaneously fueling the SMBH. Galaxy mergers also play a crucial role in fueling SMBHs in massive galaxies as SMBHs in massive halos tend to accrete a higher fraction of external gas from mergers compared to smoothly accreted gas. However, on average, it takes approximately 1.85 Gyr for external gas to enter the main galaxy and accrete onto the SMBH. Considering the presence of various other gas triggers for AGN activity alongside this time delay, the association between AGN and mergers may not always be obvious.