High resolution 21 cm observations of low column density gas clumps in the Milky Way halo
Aims. We study the properties of low column density gas clumps in the halo of the Milky Way based on high resolution 21 cm observations. Methods. Using interferometric data from the Westerbork Synthesis Radio Telescope (WSRT) and the Very Large Array (VLA) we studied H I emission at low intermediate and high radial velocities along four lines of sight towards the quasars QSO J0003 2323, QSO B1331+170, QSO B0450 1310, and J081331+254503. Along these sightlines we had previously detected weak CaII and Nan I absorbers in the optical spectra of these quasars. Results. The analysis of the high resolution H data reveals several compact and cold clumps of neutral gas at velocities similar to the optical absorption. The clumps have narrow H I line widths in the range of 1.8 u FWHM 13.0 km s 1 yielding upper limits for the kinetic temperature of the gas of 70 T max 3700 K. The neutral gas has low H I column densities in the range of 5 x 10 18 3 x 10 19 cm 2 All clumps have angular sizes of only a few arcminutes. Conclusions. Our high resolution 21 cm observations indicate that many of the Ca and Na absorbers seen in our optical quasar spectra are associated with low column density HI clumps on small angular scales. This suggests that, next to the massive, high column density neutral gas clouds in the halo (the common 21 cm low intermediate and high velocity clouds, LVCs, IVCs, and HVCs) a population of low mass, neutral gas structures exists in the halo and remain mostly unseen in the existing 21 cm all sky surveys of IVCs and HVCs. One of our absorbers may be associated with the Magellanic Stream, two intermediate velocity clouds are probably part of the Intermediate Velocity Spur and the Low latitude IV arch, respectively. The remaining systems could be located either in the lower halo or in the disk of the Milky Way. The estimated thermal gas pressures of the detected H I clumps are consistent with what is expected from theoretical models of gas in the inner and outer Milky Way halos.