Color-Magnitude Diagrams of Resolved Stars in Virgo Cluster Dwarf   Galaxies
  The Advanced Camera for Surveys (ACS) on HST has been used to image two fields in the core of the Virgo cluster that contain a number of dwarf elliptical galaxies. The combined F555W and F814W images have resolved red giant stars in these galaxies, down to 1 mag below the giant branch tip. Two of the galaxies were targeted because of their extremely low central surface brightnesses (Bo > 27.0), thus the successful resolution into stars confirms the existence of such tenuous galaxies. Red giant stars were also found that are not ostensibly associated with any galaxy. Color-magnitude diagrams in V and I have been derived and used to derive distances and metallicities via the magnitude and mean color of the red giant branch tip. The mean abundances of stars in the dwarfs range from -1.2 [Fe/H] < -2.4, and fall along the relation between galaxy luminosity and metallicity found for Local Group and M81 group dwarf ellipticals. The mean distance modulus of the six Virgo galaxies is 31.0 +/- 0.05, or 16.1 +/- 0.4 Mpc, whereas that for the intracluster stars in those fields is 31.2 +/- 0.09 (17.4 +/- 0.7 Mpc). 
Nelson Caldwell (Smithsonian Astrophysical Observatory)