The dwarf LSB galaxy population of the Virgo cluster - I. The faint-end slope of the luminosity function

The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very low surface brightness (LSB) features in deep CCD data. We apply this procedure to large-area CCD survey fields of the Virgo cluster. We show that there are many more faint (-10greater than or equal toM(B)greater than or equal to-14) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement, the faint-end slope of the luminosity function becomes alpha=-1.6. The luminosity function is flatter in the inner regions of the cluster than it is in the outer regions. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies.