Direct observational evidence that higher-luminosity type 1 active galactic nuclei are most commonly triggered by galaxy mergers

TL;DR

This study provides direct observational evidence that galaxy mergers are the primary trigger for high-luminosity type 1 AGNs in the local universe, with merger signatures increasing with AGN luminosity and black hole mass.

Contribution

It demonstrates a clear correlation between merger features and AGN luminosity/mass, highlighting mergers as the main trigger for luminous AGNs, using deep imaging data.

Findings

Higher merger feature fraction in more luminous AGNs

Merger signatures increase with black hole mass

Mergers are the dominant trigger for high-luminosity AGNs

Abstract

We examine the connection between galaxy mergers and the triggering of active galactic nuclei (AGNs) using a sample of 614 type 1 AGNs at $z<0.07$, along with a control sample of inactive galaxies matched to the AGNs for comparison. We used tidal features, detected in deep images from the DESI Legacy Imaging Survey, as direct evidence of recent mergers. We find that the fraction of type 1 AGN hosts with tidal features ($f_T$) is higher for AGNs with higher luminosities and (to a lesser extent) more massive black holes. Specifically, $f_T$ rapidly increases from $0.05\pm0.03$ to $0.75\pm0.13$ as the luminosity of the $[\mathrm{O\,{\scriptstyle III}}]$ $\lambda$5007 emission line ($L_{[\mathrm{O\,{\scriptstyle III}}]}$), an indicator for bolometric AGN luminosity, increases in the range $10^{39.5}\lesssim L_{[\mathrm{O\,{\scriptstyle III}}]}/(\mathrm{erg\,s}^{-1}) \lesssim10^{42.5}$. In addition, $f_T$ increases from $0.13\pm0.03$ to $0.43\pm0.09$ as black hole mass ($M_\mathrm{BH}$) increases in the range $10^{6.0}\lesssim M_\mathrm{BH}/M_{\odot}\lesssim10^{8.5}$. The fraction $f_T$ also increases with the Eddington ratio, although the trend is less significant compared to that with $L_{[\mathrm{O\,{\scriptstyle III}}]}$ and $M_\mathrm{BH}$. The excess of $f_T$, defined as the ratio of $f_T$ for AGNs to that of their matched inactive counterparts, exhibits similar trends, primarily increasing with $L_{[\mathrm{O\,{\scriptstyle III}}]}$ and weakly with $M_\mathrm{BH}$. Our results indicate that, in the local Universe, galaxy mergers are the predominant triggering mechanism for high-luminosity AGNs, whereas they play a lesser role in triggering lower-luminosity AGNs. Additionally, strong events, such as galaxy mergers, may be more necessary to activate massive black holes in more massive galaxies due to their lower gas fractions.