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