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

We have employed FORS1 and 2 at the Very Large Telescope at ESO to acquire deep B and R-band CCD images of 16 dwarf elliptical galaxies in the direction of the Virgo cluster. For each dwarf we measure the apparent Rband surface brightness fluctuation (SBF) magnitude mR and the (B−R)0 colour in a number of fields at different galactocentric distances. From the field-to-field variation of the two quantities we determine the SBF distance by means of the (B − R)0 − MR relation. The derived distances of the dwarfs are ranging from 14.9 Mpc to 21.3 Mpc, with a mean 1σ uncertainty of 1.4 Mpc or 8% of the distance, confirming that there is considerable depth in the distance distribution of early-type cluster members. For VCC1104 (IC3388) our SBF distance modulus of (m − M)SBF = 31.15± 0.19 (17.0± 1.5 Mpc) is in good agreement with the Harris et al. (1998) result of (m − M)TRGB = 30.98±0.19 mag (15.7±1.5 Mpc) based on HST observations and the tip magnitude of the red giant branch. Combining our results with existing distances for giant Virgo ellipticals we identify two major galaxy concentrations in the distance distribution: a broad primary clump around (M −m) = 31.0 mag (15.8 Mpc) and a narrow secondary clump around 31.33 mag (18.5 Mpc). An adaptive kernel analysis finds the two concentrations to be significant at the 99% (2.5σ) and 89% (1.6σ) levels. While the near-side clump of Virgo early-type galaxies can be associated to the subcluster centered on M87, the second clump is believed to be mainly due to the backside infalling group of galaxies around M86. The ages and metallicities of the dE stellar populations are estimated by combining the observed (B − R)0 colours with Worthey’s stellar population synthesis models. It appears that the Virgo dEs cover a wider range in metallicity, from [Fe/H]≈ −1.4 (VCC0815) to −0.5 (NGC4415), than Fornax cluster dEs. The derived metallicities place the Virgo dEs on the extension of the metallicity–luminosity relation defined by the lowluminousity Local Group dEs. The data further suggest an age range from genuinly old (∼ 17 Gyrs) stellar systems like IC3019 and IC0783 to intermediate-age (8 − 12 Gyrs) dwarfs like NGC4431 and IC3468. Subject headings: galaxies: clusters: individual (Virgo) – galaxies: distances and redshifts – galaxies: dwarf – galaxies: elliptical and lenticular, cD – galaxies: individual (IC3328, IC3388) – galaxies: stellar content