The low surface brightness extent of the Fornax cluster

We have used a large format CCD camera to survey the nearby Fornax cluster and its immediate environment for low-luminosity low surface brightness gal axies. Recent observations indicate that these are the most dark matter-dom inated galaxies known, and so they are likely to be a good tracer of the da rk matter in clusters. We have identified large numbers of these galaxies c onsistent with a steep faint-end slope of the luminosity function (alpha ap proximate to -2) down to M-B approximate to -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 t wo of the important predictions of N-body hierarchical simulations of struc ture formation using dark halos. The luminosity (mass?) function is steep, and the mass distribution is more extended than that defined by the brighte r galaxies. We also find a large concentration of low surface brightness ga laxies around the nearby galaxy NGC 1291.