On the Nature of Sodium Excess in Nearby Galaxies

Abstract

The 5890, 5895Å Na D absorption lines are sometimes found to be strongly enhanced in nearby galaxies, compared to what is predicted by the best-fitting models for the stellar continuum. The resulting Na D absorption excess could be caused either by intrinsically stronger absorption feature in the stellar continuum, due to stellar-population effects that are unaccounted by current population synthesis models, interstellar absorption or both. Using the sample of Jeong et al. (2013) and modelling the excess Na D absorption by means of physically motivated line-profile we derived the kinematics of such an excess absorption. Further drawing from the properties of the host galaxies we address the origin of such an enhanced Na absorption. In the galaxies, where no ionised-gas emission is detected and no reddening by dust is present, the Na D spectral profile is observed to be broader than the stellar line-of-sight velocity distribution, suggesting that most likely in these galaxies the Na D excess arise from stellar photospheric absorption. Conversely, objects with an Na D excess profile narrower than the stellar photospheric lines and exhibiting some reddening by dust have also a small red shift associated to their kinematics, a behaviour that can be explained in terms of interstellar absorption occurring in dust lanes that are not always perfectly settled. In objects with a strong reddening by dust and ionised-gas emission, indicating the presence of a considerable interstellar medium, we can find Na D excess profiles that are clearly blue-shifted, an indication for outflowing cold-gas. In our sample, we found that the objects, hosting outflows, are most likely generally, showing features of intense star-formation activity, or are with nebular emission, powered by a mix of star-forming and nuclear activity, with the latter sometimes attributed to accretion onto a supermassive black hole. Per se, the presence of an active galactic nucleus was found not to be relevant in driving such outflows.