Using VLT/FORS2 spectroscopy , we have studied the properties of the central ( inner 320 pc ) stellar populations of a sample of 26 nucleated early-type dwarf ( dE ) galaxies in the Virgo Cluster of magnitude range : -18.59 \leqslant M _ { \emph { r } } \leqslant -15.39 mag .
With the addition of the data of Michielsen et al .
( 2008 ) , extending the sample to 38 dEs , we find that these galaxies do not exhibit the same average stellar population characteristics for different morphological subclasses .
The nucleated galaxies without discs , which are , on average , fainter than dEs with discs and distributed in regions of higher local density , are older and more metal poor ( mean ages 7.5 \pm 1.89 Gyr and 3.1 \pm 0.83 Gyr , mean [ Z/H ] = - 0.54 \pm 0.14 dex and - 0.31 \pm 0.10 dex , respectively ) .
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 ( M _ { r } \leq - 16.5 mag ) , including the discy ones , exhibit significantly younger ages than fainter dEs ( M _ { r } \geq - 17.0 mag ) .
The magnitude overlap between these two subgroups appears to be resolved when considering in addition the weak correlation between age and local density , such that older galaxies at a given magnitude are located at higher densities .
Thus there seems to be no significant difference between the stellar populations of dEs with and without discs when compared at the same magnitude and density .
We thereby revisit the question whether both could belong to the same intrinsic population , with discs surviving only in lower-density regions .
However , 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 .