The luminosity function of the Virgo Cluster from M_B=-22 to M_B=-11

Abstract

We measure the galaxy luminosity function (LF) for the Virgo Cluster between blue magnitudes M_B = -22 and M_B = -11 from wide-field CCD imaging data. The LF is only gradually rising for -22 < M_B < -16. Between M_B = -16 and M_B = -14 it rises steeply, with a logarithmic slope alpha ~ -1.6. Fainter than M_B = -14, the LF flattens again. This LF is shallower (although turning up at brighter absolute magnitudes) than the R-band LF recently measured by Phillipps et al. (1998), who found alpha ~ -2.2 fainter than M_R = -13. It is similar, however, to the LF determined from the Virgo Cluster Catalog by Sandage et al. (1985). A few faint galaxies are found which Sandage, et al. missed because their surface-brightness threshold for detection was too high, but these do not dominate the luminosity function at any magnitude. Most of the faint galaxies we find are dwarf elliptical, alternatively called dwarf spheroidal, galaxies. The most important potential source of systematic error is that we may have rejected some high surface-brightness galaxies from the cluster sample because we think that they are background galaxies. This is quite different from what has conventionally been regarded as the most serious source of systematic error in this kind of study: that we are missing many LOW surface-brightness galaxies because they are never visible above the sky.