Photoionization Modeling of the Low Luminosity Seyfert 1 Nucleus in NGC
  3516
  Spectroscopic observations of the low luminosity Seyfert 1 nucleus in NGC
3516 obtained with the Hubble Space Telescope show that the visible spectrum is
dominated by the Balmer emission lines of Hydrogen (H) and a continuum
luminosity that rises into the UV. The anomalous H${\alpha}$/H${\beta}$
emission line ratio, the Balmer emission line luminosity and the distinctive
shape observed for the H${\alpha}$ emission line profile serve as important
constraints in any photoionization model aimed at explaining the visible
emission line spectrum of NGC 3516. Photoionization modeling using Cloudy
demonstrates that the central UV-X-ray source is able to completely ionize the
H gas in between the Balmer and dust reverberation radii if the electron
density is ${\le}$ 3 ${\times}$ 10${^7}$ cm${^{-3}}$ throughout. Thus,
according to this model the region responsible for producing the visible H
lines is a dust free shell of ionized H gas. Interestingly, the model predicts
a rapid rise in the electron temperature as the central UV-X-ray source is
approached, mirrored by an equally precipitous decrease in the Balmer line
emissivity that coincides with the Balmer reverberation radius, providing a
natural explanation for the finite width observed for the H Balmer lines.
Collectively, the merit of the model is that it explains the relative
intensities of the three brightest Balmer lines, and the shape of the
H${\alpha}$ emission line profile. However, questions remain concerning the
unusually weak forbidden lines that can not be addressed using Cloudy due to
limitations with the code.
