High-resolution 21-cm observations of low-column density gas clumps in
  the Milky Way halo
  We study the properties of low-column density gas clumps in the halo of the
Milky Way based on high-resolution 21-cm observations.
  Using interferometric data from the WSRT and the VLA we study HI emission at
low-, intermediate- and high radial velocities along four lines of sight
towards quasars. Along these sightlines we previously detected weak CaII and
NaI absorbers in their optical spectra.
  The analysis of the high-resolution HI data reveals the presence of several
compact and cold clumps of neutral gas at velocities similar to the optical
absorption. The clumps have narrow HI line widths in the range of 1.8 to 13
km/s, yielding upper limits for the kinetic temperature of the gas of 70 to
3700 K. The neutral gas has low HI column densities in the range of 5E18 to
3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes.
  Our high-resolution 21-cm observations indicate that many of the CaII and NaI
absorbers seen in our optical quasar spectra are associated with low-column
density HI clumps at small angular scales. This suggests that next to the
massive, high-column density neutral gas clouds in the halo (the common 21-cm
LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas
structures in the halo that remain mostly unseen in the existing 21-cm all-sky
surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected
HI clumps are consistent with what is expected from theoretical models of gas
in the inner and outer Milky Way halo.
