Stellar populations of Virgo cluster early-type dwarf galaxies with and
  without discs: a dichotomy in age?
  [Abridged] Using VLT/FORS2 spectroscopy, we have studied the properties of
the central stellar populations of a sample of 38 nucleated early-type dwarf
(dE) galaxies in the Virgo Cluster. We find that these galaxies do not exhibit
the same average stellar population characteristics for different morphological
subclasses. The nucleated galaxies without discs are older and more metal poor
than the dEs with discs . The alpha-element abundance ratio appears consistent
with the solar value for both morphological types. Besides a well-defined
relation of metallicity and luminosity, we also find a clear anti-correlation
between age and luminosity. More specifically, there appears to be a
bimodality: brighter galaxies, including the discy ones, exhibit significantly
younger ages than fainter dEs. Therefore, it appears less likely that fainter
and brighter dEs have experienced the same evolutionary history, as the
well-established trend of decreasing average stellar age when going from the
most luminous ellipticals towards low-luminosity Es and bright dEs is broken
here. The older and more metal-poor dEs could have had an early termination of
star formation activity, possibly being "primordial" galaxies in the sense that
they have formed along with the protocluster or experienced very early infall.
By contrast, the younger and relatively metal-rich brighter dEs, most of which
have discs, might have undergone structural transformation of infalling disc
galaxies.
