Method for Determining AGN Accretion Phase in Field Galaxies

TL;DR

This paper presents a method to determine the AGN accretion phase in field galaxies by linking observed AGN activity to galaxy merger history through cosmological simulations, explaining the evolution of AGN host properties.

Contribution

It introduces a novel approach combining cosmological N-body simulations with observational data to distinguish between recent and old merger-triggered AGNs in different galaxy types.

Findings

AGN activity in quiescent galaxies at z<1 is explained by old mergers.

The AGN occupation fraction follows galaxy merger rate evolution.

At higher redshifts, the proportion of recent and old merger AGNs becomes similar.

Abstract

Recent observations of AGN activity in massive galaxies (log Mstar / Msun > 10.4) show that: 1) at z < 1, AGN-hosting galaxies do not show enhanced merger signatures compared to normal galaxies, 2) also at z < 1, most AGNs are hosted by quiescent galaxies; and 3) at z > 1, percentage of AGNs in star forming galaxies increases and becomes comparable to AGN percentage in quiescent galaxies at z ~ 2. How can major mergers explain AGN activity in massive quiescent galaxies which have no merger features and no star formation to indicate recent galaxy merger? By matching merger events in a cosmological N-body simulation to the observed AGN incidence probability in the COSMOS survey, we show that major merger triggered AGN activity is consistent with the observations. By distinguishing between "peak" AGNs (recently merger triggered and hosted by star forming galaxies) and "faded" AGNs (merger triggered a long time ago and now residing in quiescent galaxies), we show that the AGN occupation fraction in star forming and quiescent galaxies simply follows the evolution of the galaxy merger rate. Since the galaxy merger rate drops dramatically at z < 1, the only AGNs left to be observed are the ones triggered by old mergers and are now in the declining phase of their nuclear activity, hosted by quiescent galaxies. As we go toward higher redshifts the galaxy merger rate increases and the percentages of "peak" AGNs and "faded" AGNs become comparable.