Probing the physics of narrow-line regions of Seyfert galaxies, I. The case of NGC 5427

Abstract

Context. The spectra of the extended narrow-line regions (ENLRs) of Seyfert 2 galaxies probe the physics of the central active galaxy nucleus (AGN), since they encode the energy distribution of the ionising photons, the radiative flux and radiation pressure, nuclear chemical abundances and the mechanical energy input of the (unseen) central AGN.

Aims. We aim to constrain the chemical abundance in the interstellar medium of the ENLR by measuring the abundance gradient in the circum-nuclear H ii regions to determine the nuclear chemical abundances, and to use these to in turn determine the EUV spectral energy distribution for comparison with theoretical models.

Methods. We have used the Wide Field Spectrograph (WiFeS) on the ANU 2.3 m telescope at Siding Spring to observe the nearby, nearly face-on, Seyfert 2 galaxy, NGC 5427. We have obtained integral field spectroscopy of both the nuclear regions and the H ii regions in the spiral arms. The observed spectra have been modelled using the MAPPINGS IV photoionisation code, both to derive the chemical abundances in the H ii regions and the Seyfert nucleus, and to constrain the EUV spectral energy distribution of the AGN illuminating the ENLR.

Results. We find a very high nuclear abundance, 3.0 times solar, with clear evidence of a nuclear enhancement of N and He, possibly caused by massive star formation in the extended (~100 pc) central disk structure. The circum-nuclear narrow-line region spectrum is fit by a radiation pressure dominated photoionisation model model with an input EUV spectrum from a Black Hole with mass 5 × 107 M⊙ radiating at ~0.1 of its Eddington luminosity. The bolometric luminosity is closely constrained to be log Lbol = 44.3 ± 0.1 erg s-1. The EUV spectrum characterised by a soft accretion disk and a harder component extending to above 15 keV. The ENLR region is extended in the NW-SE direction. The line ratio variation in circum-nuclear spaxels can be understood as the result of mixing H ii regions with an ENLR having a radius-invariant spectrum.