Do gas clouds in narrow-line regions of Seyfert galaxies come from their nuclei?

Abstract

Abstract The narrow-line region (NLR) consists of gas clouds ionized by the strong radiation from the active galactic nucleus (AGN), distributed in the spatial scale of AGN host galaxies. The strong emission lines from the NLR are useful to diagnose physical and chemical properties of the interstellar medium in AGN host galaxies. However, the origin of the NLR is unclear; the gas clouds in NLRs may originally be in the host and photoionized by the AGN radiation, or they may be transferred from the nucleus with AGN-driven outflows. In order to study the origin of the NLR, we systematically investigate the gas density and velocity dispersion of NLR gas clouds using a large spectroscopic data set taken from the Sloan Digital Sky Survey. The [S ii] emission-line flux ratio and [O iii] velocity width of 9571 type-2 Seyfert galaxies and 110041 star-forming galaxies suggest that the gas density and velocity dispersion of NLR clouds in Seyfert galaxies (ne ≈ 194 cm−3 and $\sigma _{\rm [O\,{\small III}]} \approx 147\:$km s−1) are systematically larger than those of clouds in H ii regions of star-forming galaxies (ne ≈ 29 cm−3 and $\sigma _{\rm [O\,{\small III}]} \approx 58\:$km s−1). Interestingly, the electron density and velocity dispersion of NLR gas clouds are larger for Seyfert galaxies with a higher [O iii]/Hβ flux ratio, i.e., with a more active AGN. We also investigate the spatially-resolved kinematics of ionized gas clouds using the Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA) survey data for 90 Seyfert galaxies and 801 star-forming galaxies. We find that the velocity dispersion of NLR gas clouds in Seyfert galaxies is larger than that in star-forming galaxies at a fixed stellar mass, at both central and off-central regions. These results suggest that gas clouds in NLRs come from the nucleus, probably through AGN outflows.