2015MNRAS.452..511M

We map out calcium II and 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 deg2 and make use of about 300000 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 Nai with respect to neutral hydrogen stays roughly constant in different environments, while that of CaII decreases with hydrogen column density. Studying how these tracers vary as a function of velocity, we show that, on average, the NNai/NCaII 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 CaII and Nai absorption needs to be taken into account for precision photometry and, more importantly, for photometric redshift estimation with star-forming galaxies. Our maps of CaII and Nai absorption are publicly available.