Small HI Clouds in Nearby Groups

Here I am considering nearby groups like the Sculptor group (2.5 Mpc) and the Centaurus A group (3.5 Mpc). These groups are dynamically unevolved and may be coming together for the first time. Table 3 gives the mean radius in Mpc and the mean absolute peculiar velocity in km of the galaxies in each group, and the crossing time t in years. This compilation includes the bright galaxies and the many dwarf galaxies discovered in each group by Côté (1995).

TABLE 3: DYNAMICAL PARAMETERS FOR NEARBY GROUPS


The interest of groups like Scl and Cen A is that, in the usual picture of galaxy formation by secondary infall, the smallest mass elements are expected to have masses of about the Jeans mass at recombination ( M). These are the basic building blocks of galaxies and are likely to be made of gas and dark matter. It seems possible that dynamically unevolved systems like these two groups, with their long crossing times, could contain significant numbers of these small primordial mass elements which have so far escaped capture by the larger galaxies. It is possible that we may be able to detect such objects directly in the multibeam survey, and measure their dark matter content dynamically. I find this an exciting prospect. If these small primordial objects are indeed present in the Scl group, then their M values may be no larger than about M, and integration times rather longer than 5 minutes will be needed to detect them.

A preliminary sparse survey of the Scl group, by Haynes and Roberts (1979), did not detect most of Côté's dwarfs (which have HI masses M), but did detect several HI clouds. The authors argued that these clouds may be associated with the Magellanic Stream rather than with the Sculptor group itself. We note however that the cloud velocities do appear to be generally somewhat higher than the Magellanic Stream velocities in this direction.

© Copyright Astronomical Society of Australia 1997