21 cm Searches for Dim Galaxies

We review very strong selection effects which operate against the detection of dim (i.e. low surface brightness) galaxies. The Parkes multibeam instrument offers a wonderful opportunity to turn up new populations of such galaxies. However, to explore the newly accessible parameter space, it will be necessary to survey both a very deep patch (10 5 s/pointing, limiting N hi ∼ 10 18 cm −2 ) and a deep patch (10 4 s/pointing, limiting N hi ∼ 3 × 10 18 cm −2 ) in carefully selected areas, and we outline the case to do this.