The fraction of AGN in major merger galaxies and its luminosity dependence

TL;DR

This study models the relationship between galaxy mergers and AGN activity at z~0, showing that merger galaxies host a higher fraction of AGN but do not necessarily have higher Eddington ratios, aligning with observed luminosity functions.

Contribution

It introduces a phenomenological model linking galaxy mergers to AGN triggering, quantifying the increased AGN fraction in merger galaxies without assuming higher Eddington ratios.

Findings

AGN fraction in merger galaxies is about 10 times higher than in the general galaxy population.

The increase in AGN fraction with luminosity is explained by the mass dependence of mergers.

Matching observed luminosity functions requires only a modest increase in Eddington ratios among merger hosts.

Abstract

We use a phenomenological model which connects the galaxy and AGN populations to investigate the process of AGN triggering through major galaxy mergers at z~0. The model uses stellar mass functions as input and allows the prediction of AGN luminosity functions based on assumed Eddington ratio distribution functions (ERDFs). We show that the number of AGN hosted by merger galaxies relative to the total number of AGN increases as a function of AGN luminosity. This is due to more massive galaxies being more likely to undergo a merger and does not require the assumption that mergers lead to higher Eddington ratios than secular processes. Our qualitative analysis also shows that to match the observations, the probability of a merger galaxy hosting an AGN and accreting at a given Eddington value has to be increased by a factor ~10 relative to the general AGN population. An additional significant increase of the fraction of high Eddington ratio AGN among merger host galaxies leads to inconsistency with the observed X-ray luminosity function. Physically our results imply that, compared to the general galaxy population, the AGN fraction among merger galaxies is ~10 times higher. On average, merger triggering does however not lead to significantly higher Eddington ratios.