An [O III] search for extended emission around AGN with H I mapping: a
  distant cloud ionized by Mkn 1
  Motivated by the discovery of large-scale ionized clouds around AGN host
galaxies, and particularly the large fraction of those which are consistent
with photoionized gaseous tidal debris, we have searched for [O III] emission
over wide fields around a set of Seyfert galaxies previously mapped in H I,
many of which show extended gas features. The detection threshold was designed
to reach mean emission-line surface brightness 10 times fainter than seen in
such AGN-ionized clouds as Hanny's Voorwerp, so that similar structures at
larger distances (and ages) could be detected. Of 24 Seyfert galaxies, we find
one extended emission feature, a discrete cloud projected 12 kpc from the
center of Mkn 1 and spanning a transverse extent of 8 kpc. Optical spectroscopy
of the Mkn 1 cloud confirms its redshift association with the Mkn 1- NGC 451
galaxy pair, shows it to closely match the kinematics of nearby H I, and
reveals emission-line ratios requiring photoionization by the AGN at roughly
the direct observed luminosity of the nucleus. Given the small fraction of H I
features with detected [O III] emission, we constrain the typical opening angle
of ionization cones in Seyfert galaxies to be of order 20 deg, if active
episodes are long compared to the light-travel times involved. An appendix
presents a derivation of an analytical expression for the probability of
intersection of a cone with randomly oriented arcs, approximating the geometry
of H I clouds and tails exposed to ionization cones. For the entire sample, the
full opening angle of bicones must be <20 deg if the AGN are continuously
bright for scales longer than the light-travel times across the H I structures.
Since many ionization cones are observed to be much broader than this, our low
detection fraction may add to evidence for the ubiquity of strong variations in
AGN luminosity on scales 10,000-100,000 years.
