Hidden Galaxies in the Fornax Cluster

We are using the Multibeam 21cm receiver on the Parkes Telescope combined with the optical Two degree Field spectrograph (2dF) of the Anglo-Australian Telescope to obtain the first complete spectroscopic sample of the Fornax cluster. In the optical the survey is unique in that all objects (both stars'' and galaxies’‘) within our magnitude limits (Bj=16.5 to 19.7) are measured, producing the most complete survey of cluster members irrespective of surface brightness. We have detected two new classes of high surface brightness dwarf galaxy in the cluster. With 2dF we have discovered a population of very low luminosity (Mb approx -12) objects which are unresolved from the ground and may be the stripped nuclei of dwarf galaxies; they are unlike any known galaxies. In a survey of the brighter (Bj=16.5 to 18) galaxies with the FLAIR-II spectrograph we have found a number of new high surface brightness dwarf galaxies and show that the fraction of star-forming dwarf galaxies in the cluster is about 30 per cent, about twice that implied by earlier morphological classifications. Our radio observations have greatly improved upon the sensitivity of the standard Multibeam survey by using a new ``basket weave’’ scanning pattern. Our initial analysis shows that we are detecting new cluster members with HI masses of order 10-to-the-8 Msun and HI mass-to-light ratios of 1-2 Msun/Lsun.