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N. Caldwell (SAO), T.A. Armandroff (KPNO)
To study the limit of central surface brightess attained by dwarf galaxies, an optical imaging survey of selected areas in the Virgo cluster has been undertaken, using the KPNO 4meter telescope and Mosaic imager. Images in B and R, covering 30 arcmin at a time were obtained of regions near the center, in the outer regions, and in a background field. We find a substantial number of ultra-low surface brightness galaxies, with central values as low as B = 27.5 mag sq. arcsec. This is nearly 2 mags fainter than the values for the galactic dwarf spheroidals, such as Draco and Umi. These Virgo galaxies are likely to be cluster members as they are not found in the background field, indeed they are rare outside of the central parts of the cluster. Their B-R colors are consistent with them being old, metal-poor stellar populations. The existence of these ultra-low SB galaxies in the center of the Virgo cluster would seem to argue for the existence of large amounts of dark matter within them. There is ample reason to expect that even lower surface brightness galaxies could be found. Confusion of small, low-surface brightness galaxies with very distant background galaxies makes it difficult to be precise about the numbers of such galaxies (no such problem exists for the large low SB galaxies), but if we limit ourselves to galaxies that are likely to be members, the luminosity function of the Virgo cluster is not very steep at the faint end (\alpha = --1.2 down to M=--10).