Color-magnitude diagrams of resolved stars in Virgo Cluster dwarf galaxies

The Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (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 (B-o > 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 for the five dwarfs, as well as the halo of a nearby spiral galaxy and the intracluster stars in the two fields. These diagrams were used to derive distances and metallicities via the magnitude of the red giant branch tip, and the mean color of the giant branch. 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 elliptical galaxies. [Fe/H] does not appear to be well-correlated with galaxy surface brightness, as the two extremely low surface brightness galaxies do not have extreme abundances. 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).