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