The host galaxies of X-ray selected Active Galactic Nuclei to z=2.5:
  Structure, star-formation and their relationships from CANDELS and
  Herschel/PACS
  We study the relationship between the structure and star-formation rate (SFR)
of X-ray selected low and moderate luminosity active galactic nuclei (AGNs) in
the two Chandra Deep Fields, using Hubble Space Telescope imaging from the
Cosmic Assembly Near Infrared Extragalactic Legacy Survey (CANDELS) and deep
far-infrared maps from the PEP+GOODS-Herschel survey. We derive detailed
distributions of structural parameters and FIR luminosities from carefully
constructed control samples of galaxies, which we then compare to those of the
AGNs. At z~1, AGNs show slightly diskier light profiles than massive inactive
(non-AGN) galaxies, as well as modestly higher levels of gross galaxy
disturbance (as measured by visual signatures of interactions and clumpy
structure). In contrast, at z~2, AGNs show similar levels of galaxy disturbance
as inactive galaxies, but display a red central light enhancement, which may
arise due to a more pronounced bulge in AGN hosts or due to extinguished
nuclear light. We undertake a number of tests of these alternatives, but our
results do not strongly favour one interpretation over the other. The mean SFR
and its distribution among AGNs and inactive galaxies are similar at z>1.5. At
z<1, however, clear and significant enhancements are seen in the SFRs of AGNs
with bulge-dominated light profiles. These trends suggest an evolution in the
relation between nuclear activity and host properties with redshift, towards a
minor role for mergers and interactions at z>1.5.
