  The cross-section of sky occupied by a particular neutral hydrogen column
density provides insight into the nature of Lyman-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_solar is compared to that
of M_star (10^10 to 10^10.5 M_solar) galaxies. We find that the column density
distribution function is dominated by low HI mass galaxies with column
densities in the range 3x10^18 to 2x10^20 cm^-2. This result is not intuitively
obvious. M_star 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_solar is
greater at moderate column densities. This result implies that moderate column
density Lyman-alpha absorption systems may be caused by a range of galaxy types
and not just large spiral galaxies as originally thought.
