Young stellar populations in early-type dwarf galaxies; occurrence,
  radial extent and scaling relations
  To understand the stellar population content of dwarf early-type galaxies
(dEs) and its environmental dependence, we compare the slopes and intrinsic
scatter of color-magnitude relations (CMRs) for three nearby clusters, Fornax,
Virgo and Coma. Additionally we present and compare internal color profiles of
these galaxies to identify central blue regions with younger stars.
  We use the imaging of the HST/ACS Fornax cluster in the magnitude range of
-18.7 <= M_g' <= -16.0, to derive magnitudes, colors and color profiles, which
we compare with literature measurements.
  Based on analysis of the color profiles, we report a large number of dEs with
young stellar populations in their center in all three clusters. While for
Virgo and Coma the number of blue-cored dEs is found to be 85 +/- 2% and 53 +/-
3% respectively, for Fornax, we find that all galaxies have a blue core. We
show that bluer cores reside in fainter dEs, similar to the trend seen in
nucleated dEs. We find no correlation between the luminosity of the galaxy and
the size of its blue core. Moreover, a comparison of the CMRs of the three
clusters shows that the scatter in Virgo's CMR is considerably larger than in
the Fornax and Coma clusters. Presenting adaptive smoothing we show that the
galaxies on the blue side of the CMR often show evidence for dust extinction,
which strengthens the interpretation that the bluer colors are due to young
stellar populations. We also find that outliers on the red side of the CMR are
more compact than expected for their luminosity. We find several of these red
outliers in Virgo, often close to more massive galaxies. No red outlying
compact early-types are found in Fornax and Coma in this magnitude range while
we find three in the Virgo cluster. We suggest that the large number of
outliers and larger scatter found for the Virgo cluster CMR is a result of
Virgo's different assembly history.
