An absorption-selected survey of neutral gas in the Milky Way halo
  We aim at analysing systematically the distribution and physical properties
of neutral and mildly ionised gas in the Milky Way halo, based on a large
absorption-selected data set. Multi-wavelength studies were performed combining
optical absorption line data of CaII and NaI with follow-up HI 21-cm emission
line observations along 408 sight lines towards low- and high-redshift QSOs. We
made use of archival optical spectra obtained with UVES/VLT. HI data were
extracted from the Effelsberg-Bonn HI survey and the Galactic All-Sky survey.
For selected sight lines we obtained deeper follow-up observations using the
Effelsberg 100-m telescope. CaII (NaI) halo absorbers at intermediate and high
radial velocities are present in 40-55% (20-35%) of the sightlines, depending
on the column density threshold chosen. Many halo absorbers show
multi-component absorption lines, indicating the presence of sub-structure. In
65% of the cases, absorption is associated with HI 21-cm emission. The CaII
(NaI) column density distribution function follows a power-law with a slope of
-2.2 (-1.4). Our absorption-selected survey confirms our previous results that
the Milky Way halo is filled with a large number of neutral gas structures
whose high column density tail represents the population of common HI high- and
intermediate-velocity clouds seen in 21-cm observations. We find that CaII/NaI
column density ratios in the halo absorbers are typically smaller than those in
the Milky Way disc, in the gas in the Magellanic Clouds, and in damped
Lyman-alpha systems. The small ratios (prominent in particular in high-velocity
components) indicate a lower level of Ca depletion onto dust grains in Milky
Way halo absorbers compared to gas in discs and inner regions of galaxies.
