Young stellar populations in early-type dwarf galaxies: Occurrence, radial extent, and scaling relations

Aims: To understand the stellar population content of early-typedwarf galaxies (dEs) and their environmental dependence, we compare theslopes and intrinsic scatter of color-magnitude relations (CMRs) forthree nearby clusters, Fornax, Virgo, and Coma. Additionally, we presentand compare internal color profiles of these galaxies to identifycentral blue regions with younger stars. Methods: We use theimaging of the HST/ACS Fornax cluster in the magnitude range of -18.7≤ Mg' ≤ -16.0 to derive magnitudes, colors, and colorprofiles, which we compare with literature measurements from the HST/ACSVirgo and Coma Cluster Survey. We take advantage of HST accuracy toinvestigate and parameterize the (g'-z') color profiles of these dEs. Results: Based on analysis of the color profiles, we report on alarge number of dEs with young stellar populations at their center inall three clusters. While for Virgo and Coma the number of blue-coreddEs is found to be 85%±2% and 53%±3%, respectively, forFornax, we find that all galaxies have a blue core. We show that bluercores reside in fainter dEs, similar to the trend seen in nucleated dEs.We find no correlation between the luminosity of the galaxy and the sizeof its blue core. Moreover, a comparison of the CMRs of the threeclusters shows that the scatter in the CMR of Virgo is considerablylarger than in the Fornax and Coma clusters. Presenting adaptivesmoothing we show that the galaxies on the blue side of the CMR oftenshow evidence for dust extinction, which strengthens the interpretationthat the bluer colors are due to young stellar populations. We also findthat outliers on the red side of the CMR are more compact than what isexpected for their luminosity. We find several of these red outliers inVirgo, often close to more massive galaxies. No red outlying compactearly-types are found in Fornax and Coma in this magnitude range whilewe find three in the Virgo cluster. We show that the CMR of the Fornaxand Virgo clusters are slightly bluer than that of Coma. We suggest thatthe large number of outliers and larger scatter found for Virgo CMR area result of the alternative assembly history of this cluster.The tables of the measured profiles are only available at the CDS viaanonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/A94.