Photoionization Modeling of the Low Luminosity Seyfert 1 Nucleus in NGC 3516

Abstract

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.