Physical Conditions in the Emission Line Gas in the Extremely Low Luminosity Seyfert Nucleus of NGC 4395

Abstract

We have combined Hubble Space Telescope/Faint Object Spectrograph, ground-based, and Infrared Space Observatory spectra of the nucleus of NGC 4395, the least luminous and nearest known type 1 Seyfert galaxy. The spectra show emission lines from a wide range of ionization states and critical densities. We have generated multicomponent photoionization models of both the broad and narrow emission-line regions (BLR and NLR) to investigate the physical conditions in the emission-line gas and test the proposition that the source of ionization is the non-thermal continuum radiation emitted by the central source. We show that, with a minimum of free parameters, the model predictions match the observed emission-line ratios quite well. The elemental abundances appear to be subsolar, with even greater underabundance of nitrogen.From the size of the BLR predicted by the models, we estimate a central mass of a few x 10^5 solar masses, in reasonable agreement with estimates from the stellar kinematics. Finally, our results suggest that the covering factor of the emission-line gas is close to unity, and that the observed UV to X-ray continuum is absorbed by intervening NLR gas. We argue that high covering factor is responsible for the apparent flattening of the Baldwin relation in low luminosity AGN.