We have carried out a sensitive high-latitude ( |b| > 15 ^ { \circ } ) HI 21cm-line absorption survey towards 102 sources using the GMRT .
With a 3 \sigma detection limit in optical depth of \sim 0.01 , this is the most sensitive HI absorption survey .
We detected 126 absorption features most of which also have corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen .
The histogram of random velocities of the absorption features is well-fit by two Gaussians centered at V _ { lsr } \sim 0 km s ^ { -1 } with velocity dispersions of 7.6 \pm 0.3 km s ^ { -1 } and 21 \pm 4 km s ^ { -1 } respectively .
About 20 % of the HI absorption features form the larger velocity dispersion component .
The HI absorption features forming the narrow Gaussian have a mean optical depth of 0.20 \pm 0.19 , a mean HI column density of ( 1.46 \pm 1.03 ) \times 10 ^ { 20 } cm ^ { -2 } , and a mean spin temperature of 121 \pm 69 K. These HI concentrations can be identified with the standard HI clouds in the cold neutral medium of the Galaxy .
The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04 \pm 0.02 , a mean HI column density of ( 4.3 \pm 3.4 ) \times 10 ^ { 19 } cm ^ { -2 } , and a mean spin temperature of 125 \pm 82 K. The HI column densities of these fast clouds decrease with their increasing random velocities .
These fast clouds can be identified with a population of clouds detected so far only in optical absorption and in HI emission lines with a similar velocity dispersion .
This population of fast clouds is likely to be in the lower Galactic Halo .