Where to find ``HI Clouds'' Now

Some very interesting examples of HI without coincident optical emission have turned up by chance in ``off-scans'' from 21cm line studies (Schneider 1989, Giovanelli & Haynes 1989, Chengalur et al 1995, Giovanelli et al 1995), although all of these detections appear to be associated in some way with nearby visible galaxies. For example, the protogalaxy of Giovanelli & Haynes (1989) now appears in VLA observations to more closely resemble a galaxy with tidal remnant (Chengalur et al 1995) than a single large cloud. An interesting consequence of high sensitivity VLA surveys of HII galaxies and LSB dwarfs (Taylor et al 1993; Taylor et al 1996) is an apparent increased probability of finding dim HI-rich companions to the HII galaxies as compared with the LSBs. This kind of study will not be possible with the crude resolution of single-dish surveys. The tendency of small galaxies to be found in the vicinity of large ones rather than in isolated regions (cf. Szomoru et al 1996) will complicate the derivation of a HI-mass function that extends to faint masses when survey observations are made with a large beam. On the other hand, isolated HI clouds (if they exist) and HI clouds associated with early type galaxies, such as polar rings and fresh examples of the Leo Ring (Schneider 1989), should be readily identified if confusion with other galaxies in the same field is not too much of a problem.

б© Copyright Astronomical Society of Australia 1997