An HI survey of the Centaurus and Sculptor groups - Constraints on the space density of low mass galaxies

We present results of two 21-cm Hi surveys performed with the Australia Telescope Compact Array in the nearby Centaurus A and Sculptor galaxy groups. These surveys are sensitive to compact Hi clouds and galaxies with Hi masses as low as similar to 3 x 10(6) M-., and are therefore among the most sensitive extragalactic H I surveys to date. The surveys consist of sparsely spaced pointings that sample approximately 2% of the groups' area on the sky. We detected previously known group members, but we found no new Hi clouds or galaxies down to the sensitivity limit of the surveys. If the Hi mass function had a faint end slope of alpha = 1.5 below M-HI = 10(7.5) M-. in these groups, we would have expected similar to 3 new objects. Cold dark matter theories of galaxy formation predict the existence of a large number low mass dark matter sub-halos that might appear as tiny satellites in galaxy groups. Our results support and extend similar conclusions derived from previous Hi surveys that a Hi rich population of these satellites does not exist.