Formation and Evolution of Dwarf Elliptical Galaxies - II. Spatially
  resolved star-formation histories
  We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs)
comparable in mass to NGC 205, and belonging to the Fornax cluster and to
nearby groups of galaxies. Using ULySS and STECKMAP, we derive radial profiles
of the SSP-equivalent ages, metallicities and star-formation histories. The old
stellar population of the dEs, which dominates their mass, is likely coeval
with that of massive ellipticals or bulges, but the star formation efficiency
is lower. Important intermediate age (1-5 Gyr) populations, and frequently
tails of star formation until recent times are detected. These histories are
reminiscent of their lower mass dSph counterparts of the Local Group. Most
galaxies (10/16) show significant metallicity gradients, with metallicity
declining by 0.5 dex over one half-light radius on average. These gradients are
already present in the old population. The flattened (or discy), rotating
objects (6/16) have flat metallicity profiles. This may be consistent with a
distinct origin for these galaxies or it may be due to their geometry. The
central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly
increasing with radius in the vast majority of objects. The group and cluster
galaxies have similar radial gradients and star-formation histories. The strong
and old metallicity gradients place important constraints on the possible
formation scenarios of dEs. Numerical simulations of the formation of spherical
low-mass galaxies reproduce these gradients, but they require a longer time for
them to build up. A gentle depletion of the gas, by ram-pressure stripping or
starvation, could drive the gas-rich, star-forming progenitors to the present
dEs.
