We report on the detection of low-column density neutral hydrogen clumps in
the halo of the Milky Way. Using high-resolution (FWHM~7 km/s) optical spectra
obtained with the VLT/UVES spectrograph we detect narrow interstellar
absorption by CaII and NaI at high radial velocities near -150 km/s toward the
quasar PKS 1448-232 (l=335.4, b=+31.7). Follow-up HI 21cm observations with the
VLA unveil the presence of a complex of small neutral hydrogen clumps with HI
column densities <8x10^18 cm^-2. The measured HI line widths imply that the gas
is relatively cold with temperatures T<900 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 CaII absorption
at high velocities is a common phenomenon, even in directions where
high-velocity HI 21cm emission is not detected. This suggests that the Milky
Way halo contains a large number of high-velocity neutral gas clumps with low
HI column densities. If such clumps are typical for halos of spiral galaxies,
they should contribute significantly to the population of strong MgII absorbers
and Lyman-Limit Systems (LLS) seen in the circumgalactic environment of other
galaxies.Comment: 4 pages, 3 figures; accepted for publication in A&A Letter