A tomographic view of metals in the Milky Way in the context of cosmic metalliticity evolution

The Milky Way (MW) interstellar and circumgalactic gas can be studied in absorption against extragalactic sources in a tomographic fashion. Damped Ly-alpha systems (DLAs), are absorption systems defined as having a HI column density higher than 20.3, at such high HI column density they are shielded from ionization and therefore metals in the gas-phase are mostly neutral. At high redshift DLA metallicity traces the metallicity of the most UV-bright galaxies, so DLA metallicity history provides important information about Galaxy metallicity evolution. The MW is relevant because in absorption it can be identified as a DLA at z=0.

In this talk I will report a study of 209 HST/COS QSO archival spectra aimed at measuring HI column densities and metallicities in the MW. These column densities are used to (1) assess spatial variations and (2) compare the variations with DLA gas-phase metallicities from the literature. We find that the average metallicity in the MW is close to solar, as expected from models of cosmic metallicity evolution. Furthermore, the scatter is ~1 dex across all the sky. This scatter is just slightly lower than the scatter seen in DLAs at all redshifts. This similarity implies that the random DLA sightlines may be subject not only to variations according to different galaxy types or masses but also to orientation effects.

I will discuss the implications that such comparison has for metal enrichment models of the MW and DLAs.