AGN photoionization of gas in companion galaxies as a probe of AGN
  radiation in time and direction
  We consider AGN photoionization of gas in companion galaxies
(cross-ionization) as a way to sample the intensity of AGN radiation in both
direction and time, independent of the gas properties of the AGN host galaxies.
From an initial set of 212 AGN+companion systems, identified with the help of
Galaxy Zoo participants, we obtained long-slit optical spectra of 32 pairs
which were a priori likely to show cross-ionization based on projected
separation or angular extent of the companion. From emission-line ratios, 10 of
these systems are candidates for cross-ionization, roughly the fraction
expected if most AGN have ionization cones with 70-degree opening angles. Among
these, Was 49 remains the strongest nearby candidate. NGC 5278/9 and UGC 6081
are dual-AGN systems with tidal debris, complicating identification of
cross-ionization. The two weak AGN in the NGC 5278/9 system ionize gas
filaments to a projected radius 14 kpc from each galaxy. In UGC 6081, an
irregular high-ionization emission region encompasses both AGN, extending more
than 15 kpc from each. The observed AGN companion galaxies with and without
signs of external AGN photoionization have similar distributions in estimated
incident AGN flux, suggesting that geometry of escaping radiation or long-term
variability control this facet of the AGN environment. This parallels
conclusions for luminous QSOs based on the proximity effect among Lyman-alpha
absorbers. In some galaxies, mismatch between spectroscopic classifications in
the common BPT diagram and the intensity of weaker He II and [Ne V] emission
lines highlights the limits of common classifications in low-metallicity
environments.
