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 nonstellar continuum radiation emitted by the central source. We show that, with a minimum of free parameters, the model predictions match the observed emission-line intensity 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 times 10⁵ M_solar, 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 a high covering factor is responsible for the apparent flattening of the Baldwin relation in low-luminosity active galactic nuclei.

Based in part on observations made with the NASA/ESA Hubble Space Telescope. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Based in part on observations made with the Infrared Space Observatory, an ESA project with instruments funded by ESA Member States with the participation of ISAS and NASA.