Outflows in Sodium Excess Objects

TL;DR

This study investigates the origins of sodium D line excess in galaxies, distinguishing between interstellar medium contributions in late-type galaxies and stellar origins in early-type galaxies through Doppler shift analysis.

Contribution

It provides evidence that Na D excess in late-type galaxies is linked to gas outflows, while in early-type galaxies it is likely due to stellar populations, based on Doppler shift measurements.

Findings

Blueshifted Na D lines in late-type NEOs indicate gas outflows.

Early-type NEOs show no Doppler shift, suggesting stellar origin.

Na D excess mechanisms differ between galaxy types.

Abstract

van Dokkum and Conroy revisited the unexpectedly strong Na I lines at 8200 A found in some giant elliptical galaxies and interpreted it as evidence for unusually bottom-heavy initial mass function. Jeong et al. later found a large population of galaxies showing equally-extraordinary Na D doublet absorption lines at 5900 A (Na D excess objects: NEOs) and showed that their origins can be different for different types of galaxies. While a Na D excess seems to be related with the interstellar medium (ISM) in late-type galaxies, smooth-looking early-type NEOs show little or no dust extinction and hence no compelling sign of ISM contributions. To further test this finding, we measured the doppler components in the Na D lines. We hypothesized that ISM would have a better (albeit not definite) chance of showing a blueshift doppler departure from the bulk of the stellar population due to outflow caused by either star formation or AGN activities. Many of the late-type NEOs clearly show blueshift in their Na D lines, which is consistent with the former interpretation that the Na D excess found in them is related with star formation-caused gas outflow. On the contrary, smooth-looking early-type NEOs do not show any notable doppler component, which is also consistent with the interpretation of Jeong et al. that the Na D excess in early-type NEOs is likely not related with ISM activities but is purely stellar in origin.