The origin of gas in extended narrow-line regions of nearby Seyfert galaxies - I. NGC 7212

Abstract

The extended narrow-line region (ENLR) of an active galactic nucleus (AGN) is a region of highly ionized gas with a size of a few up to 15–20 kpc. When it shows a conical or biconical shape with the apexes pointing towards the active nucleus, this region is also called an ionization cone. Ionization cones are evidence of the unified model, which predicts an anisotropic escape of ionizing photons from a nucleus confined to a cone by a dusty torus. Many details about the complex structure of the ENLR still remain to be unveiled, such as for example the origin of the ionized gas. Here we present new results of a study of the physical and kinematic properties of the circumnuclear gas in the nearby Seyfert 2 galaxy NGC 7212. Medium- and high-resolution integral-field spectra and broad-band photometric data were collected and analysed in the frame of an observational campaign of nearby Seyfert galaxies, with the aim of handling the complicated issue of the origin of gas in the ENLR. This work is based on (i) the analysis of gas physical properties (density, temperature and metallicity), (ii) the analysis of emission-line ratios and (iii) the study of kinematics of gas and stars. By reconstructing the [O iii]/Hβ ionization map, we point out for the first time the presence of an ionization cone extended up to about 6 kpc, made of a large amount of low-metallicity gas, kinematically disturbed and decoupled from stars, a highly ionized component of which shows radial motions at multiple velocities, as proved by the complex profiles of the spectral lines. Since NGC 7212 is a strongly interacting triple galaxy system, gravitational effects are likely to be at the origin of the ENLR in this Seyfert galaxy.