Calcium H & K and sodium D absorption induced by the interstellar and circumgalactic media of the Milky Way

TL;DR

This study maps calcium II and sodium I absorption across the Milky Way's interstellar and circumgalactic media using SDSS spectra, revealing their distribution, relation to hydrogen and dust, and implications for astrophysical measurements.

Contribution

It provides the first large-scale absorption maps of Ca II and Na I, comparing their distributions to hydrogen and dust, and analyzes their velocity-dependent behavior and impact on photometric redshift estimation.

Findings

Na I abundance remains constant across environments.

Ca II decreases with hydrogen column density.

Na I/Ca II ratio decreases at higher velocities.

Abstract

We map out calcium II & sodium I absorption (Fraunhofer H, K & D lines) induced by both the interstellar medium and the circumgalactic medium of the Milky Way. Our measurements cover more than $9000$ deg$^2$ and make use of about $300,000$ extragalactic spectra from the Sloan Digital Sky Survey. We present absorption maps for these two species and then compare their distributions to those of neutral hydrogen and dust. We show that the abundance of Na I with respect to neutral hydrogen stays roughly constant in different environments, while that of Ca II decreases with hydrogen column density. Studying how these tracers vary as a function of velocity, we show that, on average, the N(Na I)/N(Ca II) ratio decreases at higher velocity with respect to the local standard of rest, similar to the local Routly-Spitzer effect but seen on Galactic scale. We show that it is likely caused by higher gas/dust density at lower velocity. Finally, we show that Galactic Ca II and Na I absorption needs to be taken into account for precision photometry and, more importantly, for photometric redshift estimation with star forming galaxies. Our maps of Ca II and Na I absorption are publicly available.