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 the European Southern Observatory (ESO) to acquire deep B and R-band CCD images of 16 dwarf elliptical (dE) galaxies in the direction of the Virgo cluster. For each dwarf, we measure the apparent R-band surface brightness fluctuation (SBF) magnitude (m) over bar (R) and the (B - R)(0) color 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)-(M) over bar (R) relation. The derived distances of the dwarfs range from 14.9 to 21.3 Mpc, with a mean 1sigma uncertainty of 1.4 Mpc or 8% of the distance, which confirms that there is considerable depth in the distance distribution of early-type cluster members. For VCC 1104 (IC 3388), 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 result of (m - M)(TRGB) = 30.98 +/- 0.19 mag (15.7 +/- 1.5 Mpc) based on Hubble Space Telescope (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% (2.5sigma) and 89% (1.6 sigma) levels, respectively. While the near-side clump of Virgo early-type galaxies can be associated with the subcluster centered on M87, the second clump is believed to be mainly due to the far side infalling group of galaxies around M86. The ages and metallicities of the dE stellar populations are estimated by combining the observed (B - R)(0) colors with Worthey's stellar population synthesis models. It appears that the Virgo dE galaxies cover a wider range in metallicity, from [Fe/H] approximate to -1.4 (VCC 0815) to -0.5 (NGC 4415), than Fornax cluster dEs. The derived metallicities place the Virgo dEs on the extension of the metallicity-luminosity relation defined by the low-luminosity Local Group dEs. The data further suggest an age range from genuinely old (similar to17 Gyr) stellar systems such as IC 3019 and IC 0783 to intermediate-age (8 - 12 Gyr) dwarfs such as NGC 4431 and IC 3468.