Young, metal-enriched cores in early-type dwarf galaxies in the Virgo
  cluster based on colour gradients
  Early-type dwarf galaxies are not simply featureless, old objects, but were
found to be much more diverse, hosting substructures and a variety of stellar
population properties. To explore the stellar content of faint early-type
galaxies, and to investigate in particular those with recent central star
formation, we study colours and colour gradients within one effective radius in
optical (g-r) and near-infrared (i-H) bands for 120 Virgo cluster early types
with -19 mag < $M_{r}$ < -16 mag. Twelve galaxies turn out to have blue cores,
when defined as g-r colour gradients larger than 0.10 mag/$R_{\rm eff}$, which
represents the positive tail of the gradient distribution. For these galaxies,
we find that they have the strongest age gradients, and that even outside the
blue core, their mean stellar population is younger than the mean of ordinary
faint early types. The metallicity gradients of these blue-cored early-type
dwarf galaxies are, however, in the range of most normal faint early types,
which we find to have non-zero gradients with higher central metallicity. The
blue central regions are consistent with star formation activity within the
last few 100 Myr. We discuss that these galaxies could be explained by
environmental quenching of star formation in the outer galaxy regions, while
the inner star formation activity has continued.
