Correlation between Galaxy Mergers and Luminous AGN

TL;DR

This study investigates the link between galaxy mergers and luminous active galactic nuclei (AGNs), finding a significantly higher merger rate in AGN hosts compared to normal galaxies, supporting the hypothesis that mergers trigger AGN activity.

Contribution

It provides observational evidence that luminous AGNs are more frequently associated with galaxy mergers than normal early-type galaxies, using deep optical imaging and image simulations.

Findings

43.6% of AGN hosts show merger features

Simulated non-AGN galaxies show only 5-15% merger features

Results support galaxy merging as a trigger for luminous AGN activity

Abstract

It is not yet clear what triggers the activity of active galactic nuclei (AGNs), but galaxy merging has been suspected to be one of the main mechanisms fuelling the activity. Using deep optical images taken at various ground-based telescopes, we investigate the fraction of galaxy mergers in 39 luminous AGNs (M$_{R}\, \lesssim$ -22.6 mag) at $z \leq$ 0.3 (a median redshift of 0.155), of which the host galaxies are generally considered as early-type galaxies. Through visual inspection of the images, we find that 17 of 39 AGN host galaxies (43.6%) show the evidence for current or past mergers like tidal tails, shells, and disturbed morphology. In order to see if this fraction is abnormally high, we also examined the merging fraction of normal early-type galaxies in the Sloan Digital Sky Survey (SDSS) Strip 82 data (a median redshift of 0.04), of which the surface-brightness limit is comparable to our imaging data. To correct for the effects related to the redshift difference of the two samples, we performed an image simulation by putting a bright point source as an artificial AGN in the images of SDSS early-type galaxies and placing them onto the redshifts of AGNs. The merging fraction in this realistic sample of simulated AGNs is only $\sim 5 - 15\%$ ($1/4$ to $1/8$ of that of real AGNs). Our result strongly suggests that luminous AGN activity is associated with galaxy merging.