Distances, Metallicities, and Ages of Dwarf Elliptical Galaxies in the Virgo Cluster from Surface Brightness Fluctuations

We have employed FORS1/2+VLT to acquire deep B and R CCD images of 16 dwarfelliptical galaxies in the direction of the Virgo cluster. For each dwarf wemeasure the R-band surface brightness fluctuation (SBF) magnitude \bar{m}_R andthe (B-R) colour in a number of fields at different galactocentric distances. From the field-to-field variation of the two quantities we determine the SBFdistance by means of the (B-R)-\bar{M}_R relation. The derived distances of thedwarfs are ranging from 14.9-21.3Mpc, with a mean 1sigma uncertainty of 1.4Mpc,confirming that there is considerable depth in the distance distribution ofearly-type cluster members. For IC3388 our SBF distance of 17.0+-1.5Mpc is ingood agreement with the Harris et al. result based on HST observations and thetip magnitude of the red giant branch. Combining our results with existingdistances for giant Virgo ellipticals we identify two major galaxyconcentrations: a broad primary clump around 15.8Mpc and a narrow secondaryclump around 18.5Mpc. An adaptive kernel analysis finds the two concentrationsto be significant at the 99% and 89% levels. While the near-side clump of Virgoearly-type galaxies can be associated to the subcluster centered on M87, thesecond clump is believed to be mainly due to the backside infalling group ofgalaxies around M86. The ages and metallicities of the dE stellar populationsare estimated by combining the observed (B-R) colours with Worthey's stellarpopulation synthesis models. It appears that the Virgo dEs cover a wider rangein metallicity than Fornax cluster dEs. The derived metallicities place theVirgo dEs on the extension of the [Fe/H]-M_V relation defined by the LocalGroup dEs. The data further suggest an age range from genuinly old stellarsystems like IC3019 to intermediate-age dwarfs like NGC4431.Comment: Accepted for publication in AJ, 27 pages, 10 figures, 8 table