Abundance ratios in dwarf elliptical galaxies
  We determine abundance ratios of 37 dwarf ellipticals (dEs) in the nearby
Virgo cluster. This sample is representative of the early-type population of
galaxies in the absolute magnitude range -19.0 < Mr < -16.0. We analyze their
absorption line-strength indices by means of index-index diagrams and scaling
relations and use the stellar population models to interpret them. We present
ages, metallicities and abundance ratios obtained from these dEs within an
aperture size of Re/8. We calculate [Na/Fe] from NaD, [Ca/Fe] from Ca4227 and
[Mg/Fe] from Mgb. We find that [Na/Fe] is under-abundant with respect to solar
while [Mg/Fe] is around solar. This is exactly opposite to what is found for
giant ellipticals, but follows the trend with metallicity found previously for
the Fornax dwarf NGC 1396. We discuss possible formation scenarios that can
result in such elemental abundance patterns and we speculate that dEs have
disk-like SFH favouring them to originate from late-type dwarfs or small
spirals. Na-yields appear to be very metal-dependent, in agreement with studies
of giant ellipticals, probably due to the large dependence on the
neutron-excess in stars. We conclude that dEs have undergone a considerable
amount of chemical evolution, they are therefore not uniformly old, but have
extended SFH, similar to many of the Local Group galaxies.
