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

Abstract

Despite the common picture of an early-type dwarf galaxy (dE) as a quiescent galaxy with no star formation and little gas, we identify 23dEs that have blue central colors caused by recent or ongoing starformation in our sample of 476 Virgo Cluster dEs. In addition, 14objects that were mostly classified as (candidate) blue compact dwarfshave similar properties. Among the certain cluster members, the dEs withblue centers reach a fraction of more than 15% of the dE population atbrighter (mB =90% for all but oneobject. Therefore, the majority of these galaxies will appear likeordinary dEs within approximately one gigayear or less after the lastepisode 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 towhat has been found for dEs with disk features in Paper I of thisseries. Their projected spatial distribution shows no centralclustering, and their distribution with projected local density followsthat of irregular galaxies, indicative of an unrelaxed population. Thisis corroborated by their velocity distribution, which displays two sidepeaks characteristic of recent infall. We discuss possible formationmechanisms (ram-pressure stripping, tidally induced star formation, andharassment) that might be able to explain both the disk shape and thecentral star formation of the dEs with blue centers.