Introduction

In this paper we present results from a series of 21 cm observations of the Centaurus and Fornax southern galaxy clusters. These observations are part of our continuing program of blind searches for extremely low surface brightness galaxies with HI masses in the range -. In this program, the AT Compact Array is being used to image selected ``blank'' fields and probe the HI mass range -, whilst the Parkes radio telescope has been used to search the HI mass range -.

Searches for nearby galaxies in the 21 cm line of HI had by 1990 covered a total spectral bandwidth of only GHz (Briggs 1990), corresponding to per cent of the sky to a redshift depth of . For HI masses greater than , the contribution of void filling gas clouds to the cosmological density parameter (), is probably less than 0.1 per cent (Krumm & Brosch 1984, Fisher & Tully 1981). For HI masses less than , the contribution of HI clouds to is virtually unconstrained, although Hoffman et al. (1992) find a weak (five per cent of ) upper limit for .

Despite the number of surveys for low surface brightness, non-optical HI clouds which have returned null results, a number of very interesting objects have been discovered in HI observations. Such objects include HI 1225+01 (Giovanelli & Haynes 1989, the Virgo ``protogalaxy''), the Leo ring (Schneider 1985) and those galaxies found by Szomoru et al. (1993) in the Boötes void. However, there are at present very few candidates for gas clouds which are not associated with some optical emission.

© Copyright Astronomical Society of Australia 1997