Physical Conditions in the Seyfert Galaxy NGC 2992

Abstract

This paper presents long-slit spectral maps of the bicone-shaped extended narrow-line region (ENLR) in the Seyfert galaxy NGC 2992. We investigate the physical properties of the ENLR via emission-line diagnostics and compare the observations to shock and photoionization models for the excitation mechanism of the gas. The line ratios vary as a function of position in the ENLR, and the loci of the observed points on line ratio diagrams are shown to be most consistent with shock+precursor model grids. We consider the energetics of a nuclear ionizing source for the ENLR and perform the q-test in which the rate of ionizing photons from the nucleus is inferred from measurements of the density and ionization parameter. The q-test is shown to be invalid in the case of NGC 2992 because of the limitations of the [S II] λ6717/λ6731 density diagnostic. The excitation of the gas is shown to be broadly consistent with the kinematics, with higher [N II] λ6583/Hα present in the more dynamically active region. We also show that the pressure associated with the X-ray-emitting plasma may provide a large fraction of the pressure required to power the ENLR via shocks.