HST Imaging of Fading AGN Candidates I: Host-Galaxy Properties and
  Origin of the Extended Gas
  We present narrow- and medium-band HST imaging, with additional supporting
ground-based data, for 8 galaxies identified as hosting fading AGN. These have
AGN-ionized gas projected >10 kpc from the nucleus, and significant shortfall
of ionizing radiation between the distant gas and the AGN, indicating fading
AGN on ~50,000-year timescales. Every system shows evidence of ongoing or past
interactions; a similar sample of obscured AGN with extended ionized clouds
shares this incidence of disturbances. Several systems show multiple dust lanes
in different orientations, broadly fit by differentially precessing disks of
accreted material ~1.5 Gyr after initial arrival. The gas has lower metallicity
than the nuclei; three systems have abundances uniformly well below solar,
consistent with an origin in tidally disrupted low-luminosity galaxies, while
some systems have more nearly solar abundances (accompanied by such signatures
as multiple Doppler components), which may suggest redistribution of gas by
outflows within the host galaxies themselves. These aspects are consistent with
a tidal origin for the extended gas in most systems, although the ionized gas
and stellar tidal features do not always match closely. In contrast to clouds
near radio-loud AGN, these are dominated by rotation, in some cases in warped
disks. Outflows are important only in localized regions near some of the AGN.
In UGC 7342 and UGC 11185, luminous star clusters are seen within projected
ionization cones, potentially triggered by outflows. As in the discovery
example Hanny's Voorwerp/IC 2497, some clouds lack a strong correlation between
H-alpha surface brightness and ionization parameter, indicating unresolved fine
structure. Together with thin coherent filaments spanning several kpc,
persistence of these structures over their orbital lifetimes may require a role
for magnetic confinement. (Abridged)
