Galaxy mergers up to z < 2.5 II: AGN incidence of merging galaxies at separations of 3-15 kpc

TL;DR

This study investigates the occurrence of active galactic nuclei in merging galaxies at 0.3<z<2.5, finding similar AGN fractions in mergers and non-mergers, with star-forming mergers showing higher AGN activity, suggesting mergers influence star formation and AGN activity early on.

Contribution

It provides the first comprehensive analysis of AGN incidence in merging galaxies across a wide redshift range using multi-wavelength data and a novel merger selection method.

Findings

AGN fraction in mergers is similar to non-mergers (~16%)

Star-forming mergers have higher AGN incidence than quiescent ones

Mergers show a higher correlation with star formation than non-mergers

Abstract

We present a study of the incidence of active galactic nucleus (AGN) in a sample of major merging systems at 0.3<z<2.5. Galaxies in this merger sample have projected separations between 3 to 15 kpc and are selected from the CANDELS/3D-HST catalogs using a peak-finding algorithm. AGNs in mergers and non-mergers are identified on the basis of their X-ray emission, optical lines, mid-infrared colors, and radio emission. Among galaxies with adequate measurements to find potential AGNs, we find a similar fraction of AGNs in mergers (16.4%) compared to the fraction found in non-merging galaxies (15.4%). In mergers, this fraction is obtained by assuming that, in unresolved observations, only one of the merging galaxies is the AGN source. The similarity between the fractions is possibly due to the higher availability of cold gas at high redshifts, where the excess of nuclear activity as a result of merging is less important than at lower redshifts. Star-forming galaxies have a higher incidence of AGNs than quiescent galaxies. In particular, starbursts in mergers are the most common sites of AGN activity since they present higher AGN fractions and black hole accretion rates. We find no clear correlation between the black hole accretion rate and the galaxy properties (i.e., star-formation rate, stellar mass) in mergers and non-mergers. However, mergers seem to have a higher correlation with star formation than non-mergers, which possibly indicates that the merging process is starting to influence the star formation and AGN activity even at this pre-coalescence stage.