Morphologies of Radio, X-Ray, and Mid-Infrared Selected AGN
  We investigate the optical morphologies of candidate active galaxies
identified at radio, X-ray, and mid-infrared wavelengths. We use the Advanced
Camera for Surveys General Catalog (ACS-GC) to identify 372, 1360, and 1238 AGN
host galaxies from the VLA, XMM-Newton and Spitzer Space Telescope observations
of the COSMOS field, respectively. We investigate both quantitative (GALFIT)
and qualitative (visual) morphologies of these AGN host galaxies, split by
brightness in their selection band. We find that the radio-selected AGN are
most distinct, with a very low incidence of having unresolved optical
morphologies and a high incidence of being hosted by early-type galaxies. In
comparison to X-ray selected AGN, mid-IR selected AGN have a slightly higher
incidence of being hosted by disk galaxies. These morphological results conform
with the results of Hickox et al. 2009 who studied the colors and large-scale
clustering of AGN, and found a general association of radio-selected AGN with
``red sequence'' galaxies, mid-IR selected AGN with ``blue cloud'' galaxies,
and X-ray selected AGN straddling these samples in the ``green valley.'' In the
general scenario where AGN activity marks and regulates the transition from
late-type disk galaxies into massive elliptical galaxies, this work suggests
that the earlier stages are most evident as mid-IR selected AGNs. Mid-IR
emission is less susceptible to absorption than the relatively soft X-rays
probed by XMM-Newton, which are seen at later stages in the transition.
Radio-selected AGN are then typically associated with minor bursts of activity
in the most massive galaxies.
