Low-column density HVC and IVC gas in the halo of the Milky Way

TL;DR

This study systematically investigates low-column density neutral and ionized gas in the Milky Way's halo using absorption lines and emission data, revealing small-scale structures and their potential impact on galactic evolution.

Contribution

It provides a comprehensive analysis of low-column density gas in the Milky Way halo, combining absorption and emission data to uncover structures unseen in large surveys.

Findings

Identification of small-scale neutral and ionized gas structures.

Comparison of Milky Way halo gas distribution with quasar absorber data.

Evidence that low-column density gas may significantly influence galactic evolution.

Abstract

Recent studies of the circumgalactic gaseous environment of the Milky Way have concentrated on the distribution, chemical composition, and physical properties of the most massive neutral gas clouds and the highly-ionized halo absorbers. Relatively little effort has been put so far in exploring the circumgalactic neutral and weakly ionized metal absorbers at low HI column densities. With our work we systematically study the distribution and physical properties of neutral and ionised low-column density gas in the halo of the Milky Way. We combine CaII and NaI absorption line measurements with HI 21-cm emission line data. For some of the sight lines high-resolution radio synthesis observations were performed allowing us to study small-scale structures that cannot be resolved with single dish telescopes. In total 177 lines of sight were observed, providing a large absorption-selected data sample for the analysis of IVC and HVC gas in the circumgalactic environment of the Milky Way. The study allows us to compare the observed absorption column density distribution (CDD) of gas in the Milky Way halo with the overall CDD of intervening absorbers towards quasars. The sensitive absorption line analysis enables us to identify the neutral and ionised gaseous structures at low column densities and small angular extent that possibly remain unseen in large 21-cm all-sky surveys. If this gas cover a significant portion of the sky, it possibly has a large influence on the evolution of the Milky Way.