The Effects of an AGN on Host Galaxy Colour and Morphology Measurements
  We assess the effects of simulated active galactic nuclei (AGNs) on the
colour and morphology measurements of their host galaxies. To test the
morphology measurements, we select a sample of galaxies not known to host AGNs
and add a series of point sources scaled to represent specified fractions of
the observed V band light detected from the resulting systems; we then compare
morphology measurements of the simulated systems to measurements of the
original galaxies. AGN contributions >20 per cent bias most of the morphology
measurements tested, though the extent of the apparent bias depends on the
morphological characteristics of the original galaxies. We test colour
measurements by adding to non-AGN galaxy spectra a quasar spectrum scaled to
contribute specified fractions of the rest-frame B band light detected from the
resulting systems. A quasar fraction of 5 per cent can move the NUV-r colour of
an elliptical galaxy from the UV-optical red sequence to the green valley, and
20 per cent can move it into the blue cloud. Combining the colour and
morphology results, we find that a galaxy/AGN system with an AGN contribution
>20 per cent may appear bluer and more bulge-dominated than the underlying
galaxy. We conclude that (1) bulge-dominated, E/S0/Sa, and early-type
morphology classifications are accurate for red AGN host galaxies and may be
accurate for blue host galaxies, unless the AGN manifests itself as a
well-defined point source; and (2) although highly unobscured AGNs, such as the
quasar used for our experiments, can significantly bias the measured colours of
AGN host galaxies, it is possible to identify such systems by examining optical
images of the hosts for the presence of a point source and/or measuring the
level of nuclear obscuration.
