On the Absence of Broad Forbidden Emission Lines in the Low Luminosity Seyfert 1 Nucleus of NGC 3227

Abstract

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.