A Hubble Space Telescope Imaging Survey of Low-Redshift Swift-BAT Active
  Galaxies
  We present initial results from a Hubble Space Telescope snapshot imaging
survey of the host galaxies of Swift-BAT active galactic nuclei (AGN) at z<0.1.
The hard X-ray selection makes this sample sample relatively unbiased in terms
of obscuration compared to optical AGN selection methods. The high-resolution
images of 154 target AGN enable us to investigate the detailed photometric
structure of the host galaxies, such as the Hubble type and merging features.
We find that 48% and 44% of the sample is hosted by early-type and late-type
galaxies, respectively. The host galaxies of the remaining 8% of the sample are
classified as peculiar galaxies because they are heavily disturbed. Only a
minor fraction of host galaxies (18%-25%) exhibit merging features (e.g., tidal
tails, shells, or major disturbance). The merging fraction increases strongly
as a function of bolometric AGN luminosity, revealing that merging plays an
important role in triggering luminous AGN in this sample. However, the merging
fraction is weakly correlated with the Eddington ratio, suggesting that merging
does not necessarily lead to an enhanced Eddington ratio. Type 1 and type 2 AGN
are almost indistinguishable in terms of their Hubble type distribution and
merging fraction. However, the merging fraction of type 2 AGN peaks at a lower
bolometric luminosity compared with those of type 1 AGN. This result may imply
that the triggering mechanism and evolutionary stages of type 1 and type 2 AGN
are not identical.
