Gas Origin in the Extended Narrow Line Region of Nearby Seyfert Galaxies

Abstract

The Extended Narrow Line Region (ENLR) of Active Galactic Nuclei (AGN) is a conical or bi-conical region of highly ionized gas with a size of few up to 15–20 kpc and the apexes pointing towards the active nucleus. This region is also called ionization cones. These cones are an evidence of the Unified Model (Antonucci 1993) that predicts an anisotropic escape of ionizing photons from the nucleus confined in a conical flow by a dusty torus. Many details about the complex structure of the ENLR still remains unveiled. For example it is not clear which is the origin of the ionized gas. This is an important issue, being related to the fuelling of the active nucleus. The gas could be simply part of the interstellar medium of the host galaxy, ionized by the active nucleus or by shocks; or it could be intergalactic medium acquired by the surroundings of the host galaxy through gravitational interactions. Up to now only 25 Seyfert 2 galaxies with ionization cones are known and studied. The lack of cones in Seyfert 2 galaxies could be a selection effect, or it could be due to the absence of gas distributed over large scales in the host galaxy, but it can also indicate that some particular initial conditions are needed in order to form an ENLR. In this work we tackle the question of the origin of the gas, by combining the information obtained from IFU, longslit and echelle spectra. Being the IFU field of view typically small, in order to correctly interpret the spectroscopic results we analyse also broad band images. We reduced and analyzed the collected data from our observational campaign of nearby Seyfert galaxies. IFU spectra were taken with MPFS at SAO 6-mt telescope, longslit spectra with AFOSC at the 1.8-mt telescope of Asiago Astrophysical Observatory and the echelle spectra with MagE at 6.5-mt Magellan telescope. The broad band images were taken with SCORPIO at SAO 6-mt telescope and from the ING archive. This work is based on: (i) analysis of gas physical properties (density, temperature and metallicity), (ii) analysis of emission lines ratios, (iii) study of gas and stars kinematics, (iv) morphological analysis. We applied this method to two Seyfert galaxies showing extended [O III] emission, NGC 7212, a Seyfert 2 galaxy and MRK 6 a Seyfert 1.5 galaxy. We obtained strong proof of an external origin for the ionized gas in both cases. This is more likely to occur in NGC 7212 because it is in a strongly interacting system, however also in MRK 6 we found hints of a past merger. We took advantage of integral field data which allow to spatially sample the ENLR in 1×1 emission line sub-regions and we selected 12 Seyfert 2 galaxies to perform a statistical study of spectroscopic properties of a large number of these regions. From the analysis of about 850 sub-regions we pointed out that most of them share the properties generally observed in the NLR of Seyfert galaxies. All these results strongly suggest that particular initial conditions are mandatory in order to generate a kpc-size NLR.