2013/09/29 by Nick Devereux, Devereux, Nick
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #astro-ph.CO
paper · pdf · doi:10.48550/arxiv.1309.7623
Revised to include an important prologue
openalex publication_date 2013/09/29 · arxiv created 2015/12/22 · arxiv updated 2015/12/23 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The absence of broad [O III] λλ4959,5007 forbidden emission lines is one of the key arguments cited in the published literature for the gas density exceeding the critical density; 7 × 105 cm-3, in the broad line region (BLR) of active galactic nuclei. However, for NGC 3227, an equally valid alternative explanation is that O2+ is progressively ionized to O3+ 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.