Triggering optical AGN: the need for cold gas, and the indirect roles of galaxy environment and interactions

TL;DR

This study shows that the presence and brightness of optical AGN are mainly driven by cold gas availability in galaxy centers, with environment and interactions playing indirect roles by affecting gas supply.

Contribution

It demonstrates that galaxy environment and interactions have minimal direct impact on optical AGN activity when controlling for stellar mass and star formation.

Findings

AGN prevalence and luminosity are primarily linked to cold gas availability.

Local density and interactions have minimal direct effects on AGN activity.

Secular processes mainly drive AGN activity in most galaxies.

Abstract

We present a study of the prevalence and luminosity of Active Galactic Nuclei (AGN; traced by optical spectra) as a function of both environment and galaxy interactions. For this study we used a sample of more than 250000 galaxies drawn from the Sloan Digital Sky Survey and, crucially, we controlled for the effect of both stellar mass and central star formation activity. Once these two factors are taken into account, the effect of the local density of galaxies and of one-on-one interactions is minimal in both the prevalence of AGN activity and AGN luminosity. This suggests that the level of nuclear activity depends primarily on the availability of cold gas in the nuclear regions of galaxies and that secular processes can drive the AGN activity in the majority of cases. Large scale environment and galaxy interactions only affect AGN activity in an indirect manner, by influencing the central gas supply.