On the Absence of Broad Forbidden Emission Lines in the Low Luminosity
  Seyfert 1 Nucleus of NGC 3227
  The absence of broad [O III] ${\lambda\lambda}$4959,5007 forbidden emission
lines is one of the key arguments cited in the published literature for the gas
density exceeding the critical density; 7 ${\times}$ 10$^5$ cm$^{-3}$, in the
broad line region (BLR) of active galactic nuclei. However, for NGC 3227, an
equally valid alternative explanation is that O$^{2+}$ is progressively ionized
to O$^{3+}$ as the central UV-X-ray source is approached. Observational
evidence for such an ionization gradient is provided by spectra obtained with
the Space Telescope Imaging Spectrograph. Modeling the rich UV-visible spectrum
of NGC 3227 with the photoionization code Cloudy supports the interpretation
that the absence of broad [O III] forbidden emission lines is due to
ionization, rather than high gas density, and further suggests that what we
perceive as the BLR in NGC 3227 is just the illuminated portion of a much
larger inflow. The low metallicity deduced for the inflowing gas suggests an
origin in the circumgalactic medium.
