We have used a large format CCD camera to survey the nearby Fornax cluster and its immediate environment for low-luminosity low surface brightness galaxies. Recent observations indicate that these are the most dark matter–dominated galaxies known, and so they are likely to be a good tracer of the dark matter in clusters. We have identified large numbers of these galaxies consistent with a steep faint-end slope of the luminosity function (α ≈ -2) down to MB ≈ -12. These galaxies contribute almost the same amount to the total cluster light as the brighter galaxies, and they have a spatial extent that is some 4 times larger. They satisfy two of the important predictions of N-body hierarchical simulations of structure formation using dark halos. The luminosity (mass?) function is steep, and the mass distribution is more extended than that defined by the brighter galaxies. We also find a large concentration of low surface brightness galaxies around the nearby galaxy NGC 1291.
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