A Census of Gas Outflows in Type 2 Active Galactic Nuclei

Abstract

We perform a census of ionized gas outflows using a sample of ~23,000 type 2 active galactic nuclei (AGNs) out to z~0.1. By measuring the velocity offset of narrow emission lines, i.e., [O III] {\lambda}5007 and H{\alpha}, with respect to the systemic velocity measured from the stellar absorption lines, we find that 47% of AGNs display an [O III] line-of-sight velocity offset $\geq$ 20 km/s. The fraction of the [O III] velocity offset in type 2 AGNs is comparable to that in type 1 AGNs after considering the projection effect. AGNs with a large [O III] velocity offset preferentially have a high Eddington ratio, implying that the detected velocity offsets are related to black hole activity. The distribution of the host galaxy inclination is clearly different between the AGNs with blueshifted [O III] and the AGNs with redshifted [O III], supporting the combined model of the biconical outflow and dust obscuration. In addition, for ~3% of AGNs, [O III] and H{\alpha} show comparable large velocity offsets, indicating a more complex gas kinematics than decelerating outflows in a stratified narrow-line region.