The independence of neutral and ionized gas outflows in low-z galaxies

TL;DR

This study uses SDSS data to analyze the kinematics of neutral and ionized gas outflows in low-redshift galaxies, revealing their independence and the influence of galaxy orientation on neutral gas outflows.

Contribution

It demonstrates that neutral and ionized gas outflows are kinematically independent and unaffected by AGN activity, highlighting the role of star formation in driving neutral outflows.

Findings

Neutral gas outflows show high velocity and dispersion in some galaxies.

No correlation between neutral and ionized gas outflow kinematics.

Neutral outflows are more prominent in face-on galaxies, indicating radial, star formation-driven winds.

Abstract

Using a large sample of emission line galaxies selected from the Sloan Digital Sky Survey, we investigate the kinematics of the neutral gas in the interstellar medium (ISM) based on the Na I$\lambda\lambda$5890,5896 (Na D) doublet absorption line. By removing the Na D contribution from stellar atmospheres, we isolate the line profile of the Na D excess, which represents the neutral gas in the ISM. The kinematics traced by the Na D excess show high velocity and velocity dispersion for a fraction of galaxies, indicating the presence of neutral gas outflows. We find that the kinematics measured from the Na D excess are similar between AGNs and star-forming galaxies. Moreover, by comparing the kinematics traced by the Na D excess and those by the [O III]$\lambda$5007 line taken from Woo et al. (2017), which traces ionized outflows driven by AGNs, we find no correlation between them. These results demonstrate that the neutral gas in the ISM traced by the Na D excess and the ionized gas traced by [O III] are kinematically independent, and AGN has no impact on the neutral gas outflows. In contrast to [O III], we find that the measured line-of-sight velocity shift and velocity dispersion of the Na D excess increase for more face-on galaxies due to the projection effect, supporting that Na D outflows are radially driven (i.e., perpendicular to the major axis of galaxies), presumably due to star formation.