Low-column density gas clumps in the halo of the Milky Way

We report on the detection of low-column density neutral hydrogen clumps in the halo of the Milky Way. Using high-resolution ( km s-1) optical spectra obtained with the VLT/UVES spectrograph we detect narrow interstellar absorption by Ca ii and Na i at high radial velocities near km s-1 toward the quasar PKS 1448-232 (). Follow-up H i 21 cm observations with the VLA unveil the presence of a complex of small neutral hydrogen clumps with H i column densities < cm-2. The measured H i line widths imply that the gas is relatively cold with temperatures K. Although the line of sight towards PKS 1448-232 does not pass immediately through a known large high-velocity cloud (HVC), the sky position and the measured radial velocities suggest that these clumps are associated with HVC cloud complex L. An inspection of other UVES quasar spectra shows that weak, narrow Ca ii absorption at high velocities is a common phenomenon, even in directions where high-velocity H i 21 cm emission is not detected. This suggests that the Milky Way halo contains a large number of high-velocity neutral gas clumps with low H i column densities. If such clumps are typical for halos of spiral galaxies, they should contribute significantly to the population of strong Mg ii absorbers and Lyman-Limit Systems (LLS) seen in the circumgalactic environment of other galaxies.