CANDELS: Constraining the AGN-Merger Connection with Host Morphologies
  at z~2
  Using HST/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared
Deep Extragalactic Legacy Survey (CANDELS), we examine the role that major
galaxy mergers play in triggering active galactic nuclei (AGN) activity at z~2.
Our sample consists of 72 moderate-luminosity (Lx ~ 1E42-1E44 erg/s) AGN at
1.5<z<2.5 that are selected using the 4 Msec Chandra observations in the
Chandra Deep Field South, the deepest X-ray observations to date. Employing
visual classifications, we have analyzed the rest-frame optical morphologies of
the AGN host galaxies and compared them to a mass-matched control sample of 216
non-active galaxies at the same redshift. We find that most of the AGN reside
in disk galaxies (51.4%), while a smaller percentage are found in spheroids
(27.8%). Roughly 16.7% of the AGN hosts have highly disturbed morphologies and
appear to be involved in a major merger or interaction, while most of the hosts
(55.6%) appear relatively relaxed and undisturbed. These fractions are
statistically consistent with the fraction of control galaxies that show
similar morphological disturbances. These results suggest that the hosts of
moderate-luminosity AGN are no more likely to be involved in an ongoing merger
or interaction relative to non-active galaxies of similar mass at z~2. The high
disk fraction observed among the AGN hosts also appears to be at odds with
predictions that merger-driven accretion should be the dominant AGN fueling
mode at z~2, even at moderate X-ray luminosities. Although we cannot rule out
that minor mergers are responsible for triggering these systems, the presence
of a large population of relatively undisturbed disk-like hosts suggests that
secular processes play a greater role in fueling AGN activity at z~2 than
previously thought.
