Galaxy Zoo DESI: large-scale bars as a secular mechanism for triggering AGN

TL;DR

This study shows that large-scale galactic bars significantly increase the likelihood of hosting an active galactic nucleus (AGN), with strong bars nearly doubling the AGN probability compared to unbarred galaxies, highlighting bars' role in SMBH growth.

Contribution

It provides the first large-scale statistical evidence that strong galactic bars facilitate AGN activity, advancing understanding of secular SMBH growth mechanisms.

Findings

Strong bars increase AGN fraction to 31.6%

Weak bars have an AGN fraction of 23.3%

Unbarred galaxies have an AGN fraction of 14.2%

Abstract

Despite the evidence that supermassive black holes (SMBHs) co-evolve with their host galaxy, and that most of the growth of these SMBHs occurs via merger-free processes, the underlying mechanisms which drive this secular co-evolution are poorly understood. We investigate the role that both strong and weak large-scale galactic bars play in mediating this relationship. Using 72,940 disc galaxies in a volume-limited sample from Galaxy Zoo DESI, we analyse the active galactic nucleus (AGN) fraction in strongly barred, weakly barred, and unbarred galaxies up to z = 0.1 over a range of stellar masses and colours. After controlling for stellar mass and colour, we find that the optically selected AGN fraction is 31.6 +/- 0.9 per cent in strongly barred galaxies, 23.3 +/- 0.8 per cent in weakly barred galaxies, and 14.2 +/- 0.6 per cent in unbarred disc galaxies. These are highly statistically robust results, strengthening the tantalising results in earlier works. Strongly barred galaxies have a higher fraction of AGNs than weakly barred galaxies, which in turn have a higher fraction than unbarred galaxies. Thus, while bars are not required in order to grow a SMBH in a disc galaxy, large-scale galactic bars appear to facilitate AGN fuelling, and the presence of a strong bar makes a disc galaxy more than twice as likely to host an AGN than an unbarred galaxy at all galaxy stellar masses and colours.