Spectral properties of the narrow-line region in Seyfert galaxies selected from the SDSS-DR7

TL;DR

This study analyzes the spectral properties of the narrow-line region in a large sample of Seyfert galaxies from SDSS-DR7, revealing similarities and inclination-related differences in physical conditions and kinematics.

Contribution

It provides a comprehensive statistical analysis of NLR physical parameters in Seyfert galaxies using a large spectroscopic sample and a self-developed measurement code.

Findings

NLR properties are similar in Seyfert 2 and Intermediate Seyfert galaxies.

Lower extinction and different gas kinematics are linked to line-of-sight inclination.

Higher [O III] blue asymmetries are observed in Intermediate Seyferts.

Abstract

Although the properties of the narrow-line region (NLR) of active galactic nuclei(AGN) have been deeply studied by many authors in the past three decades, many questions are still open. The main goal of this work is to explore the NLR of Seyfert galaxies by collecting a large statistical spectroscopic sample of Seyfert 2 and Intermediate-type Seyfert galaxies having a high signal-to-noise ratio in order to take advantage of a high number of emission-lines to be accurately measured. 2153 Seyfert 2 and 521 Intermediate-type Seyfert spectra were selected from Sloan Digital Sky Survey - Data Release 7 (SDSS-DR7) with a diagnostic diagram based on the oxygen emission-line ratios. All the emission-lines, broad components included, were measured by means of a self-developed code, after the subtraction of the stellar component. Physical parameters, such as internal reddening, ionization parameter, temperature, density, gas and stellar velocity dispersion were determined for each object. Furthermore, we estimated mass and radius of the NLR, kinetic energy of the ionized gas, and black-hole accretion rate. From the emission-line analysis and the estimated physical properties, it appears that the NLR is similar in Seyfert 2 and Intermediate-Seyfert galaxies. The only differences, lower extinction, gas kinematics in general not dominated by the host galaxy gravitational potential and higher percentage of [O III]5007 blue asymmetries in Intermediate-Seyfert can be ascribed to an effect of inclination of our line of sight with respect to the torus axis.