  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 be originally in the host and photoionized by the AGN radiation, or
they may be transferred from the nucleus with AGN-driven outflows. For studying
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 9,571 type-2 Seyfert galaxies and 110,041
star-forming galaxies suggest that the gas density and velocity dispersion of
NLR clouds in Seyfert galaxies (ne ~ 194 cm-3 and sigma([O III]) ~ 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([O III]) ~ 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]/Hbeta 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.
