Outflows in Sodium Excess Objects
  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.
