The cross-section of sky occupied by a particular neutral hydrogen column density ( N _ { HI } ) provides insight into the nature of Ly \alpha absorption systems .
We have measured this column density distribution at z=0 using 21-cm Hi emission from a blind survey .
A subsample of HI Parkes All Sky Survey ( HIPASS ) galaxies have been imaged with the Australia Telescope Compact Array ( ATCA ) .
The contribution of low Hi mass galaxies ( 10 ^ { 7.5 } to 10 ^ { 8 } M _ { \odot } ) is compared to that of M _ { * } ( 10 ^ { 10 } to 10 ^ { 10.5 } M _ { \odot } ) galaxies .
We find that the column density distribution function is dominated by low Hi mass galaxies in the range 3 \times 10 ^ { 18 } < \mbox { $N _ { HI } $ } < 2 \times 10 ^ { 20 } cm ^ { -2 } .
This result is not intuitively obvious .
M _ { * } galaxies may contain the bulk of the Hi gas , but the cross-section presented by low Hi mass galaxies ( 10 ^ { 7.5 } to 10 ^ { 8 } M _ { \odot } ) is greater at moderate column densities .
This result implies that moderate N _ { HI } Ly \alpha absorption systems may be caused by a range of galaxy types and not just large spiral galaxies as originally thought .