BASS. LI. Cool gas supply of HI-massive local Seyfert galaxies

TL;DR

This study investigates how cool gas is supplied to active galactic nuclei in HI-rich Seyfert galaxies, highlighting the role of external gas exchange and galaxy interactions in fueling AGN activity.

Contribution

It provides new evidence linking extended HI disks and external gas accretion to enhanced AGN activity in local Seyfert galaxies.

Findings

Most galaxies have HI-rich neighbors with signs of gas exchange.

Extended HI disks correlate with stronger AGN activity.

External gas accretion likely fuels AGNs in these systems.

Abstract

We present neutral atomic hydrogen (HI) imaging observations of 22 HI-rich ($M_{\rm HI} \gtrsim 10^{9.7} M_\odot$), hard X-ray-selected local Seyferts to explore how cool gas is supplied to active galactic nuclei (AGN) hosts. The sample predominantly resides in group-like, gas-rich environments. About 80% (18/22) of the galaxies have HI-detected neighbors, 61% (11/18) of which clearly exhibit strong lopsidedness, one-sided gas tails, and/or gas structures connecting to nearby companion galaxies, suggesting gas exchange histories. We examine the HI size-mass relation and star formation properties of these HI-rich AGN hosts, finding no systematic deviations from known scaling relations. In most cases, our samples are the most massive systems within their respective groups, implying that our sample is more likely to acquire gas rather than lose it. Interestingly, galaxies with more extended HI disks show stronger AGN activity. Considering that extended HI is often associated with external processes, this finding suggests that environmentally accreted gas - through galaxy interactions and gas exchange with neighboring systems - may have played a role in supplying additional fuel to the AGNs in our sample. Notably, the HI extent-AGN activity correlation becomes even tighter for those AGN hosts whose neighboring galaxies are gas poor or lack HI, further supporting externally supplied gas as a fuel source.