Virgo Cluster Early?Type Dwarf Galaxies with the Sloan Digital Sky Survey. II. Early?Type Dwarfs with Central Star Formation

Despite the common picture of an early-type dwarf (dE) as a quiescent galaxywith no star formation and little gas, we identify 23 dEs that have bluecentral colors caused by recent or ongoing star formation in our sample of 476Virgo cluster dEs. In addition, 14 objects that were mostly classified as(candidate) BCDs have similar properties. Among the certain cluster members,the dEs with blue centers reach a fraction of more than 15% of the dEpopulation at brighter (B<=16) magnitudes. A spectral analysis of the centersof 16 galaxies reveals in all cases an underlying old population that dominatesthe mass, with M(old)>=90% for all but one object. Therefore the majority ofthese galaxies will appear like ordinary dEs within ~one Gigayear or less afterthe last episode of star formation. Their overall gas content is less than thatof dwarf irregular galaxies, but higher than that of ordinary dEs. Theirflattening distribution suggests the shape of a thick disk, similar to what hasbeen found for dEs with disk features in Paper I of this series. Theirprojected spatial distribution shows no central clustering, and theirdistribution with projected local density follows that of irregular galaxies,indicative of an unrelaxed population. This is corroborated by their velocitydistribution, which displays two side peaks characteristic of recent infall. Wediscuss possible formation mechanisms (ram-pressure stripping, tidally inducedstar formation, harassment) that might be able to explain both the disk shapeand the central star formation of the dEs with blue centers.