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

Abstract

We have employed FORS1/2+VLT to acquire deep B and R CCD images of 16 dwarf elliptical galaxies in the direction of the Virgo cluster. For each dwarf we measure the R-band surface brightness fluctuation (SBF) magnitude \bar{m}_R and the (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 SBF distance by means of the (B-R)-\bar{M}_R relation. The derived distances of the dwarfs 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 of early-type cluster members. For IC3388 our SBF distance of 17.0+-1.5Mpc is in good agreement with the Harris et al. result 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: a broad primary clump around 15.8Mpc and a narrow secondary clump around 18.5Mpc. An adaptive kernel analysis finds the two concentrations to be significant at the 99% and 89% 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) colours with Worthey's stellar population synthesis models. It appears that the Virgo dEs cover a wider range in metallicity than Fornax cluster dEs. The derived metallicities place the Virgo dEs on the extension of the [Fe/H]-M_V relation defined by the Local Group dEs. The data further suggest an age range from genuinly old stellar systems like IC3019 to intermediate-age dwarfs like NGC4431.