Early-type galaxies at large galactocentric radii - II. Metallicity
  gradients, and the [Z/H]--mass, [alpha/Fe]--mass relations
  We present the results of a study of stellar population properties at large
galactocentric radii of 14 low-mass early-type galaxies in the Fornax and Virgo
clusters. We derive radial profiles of Age, total metallicity [Z/H], and
[alpha/Fe] abundance ratios out to 1 - 3 effective radii by using nearly all of
the Lick/IDS absorption-line indices in comparison to recent single stellar
population models. We extend our study to higher galaxy mass via a novel
literature compilation of 37 early-type galaxies, which provides stellar
population properties out to one effective radius. We find that metallicity
gradients correlate with galactic mass, and the relationship shows a sharp
change in slope at a dynamical mass of 3.5 10^10 M_{sun}. The central and mean
values of the stellar population parameters (measured in r < r_e/8, and at r =
r_e, respectively) define positive mass trends. We suggest that the low
metallicities, almost solar [alpha/Fe] ratios and the tight mass-metallicity
gradient relation displayed by the low-mass galaxies are indicative of an early
star-forming collapse with extended (i.e., > 1 Gyr), low efficiency star
formation, and mass-dependent galactic outflows of metal-enriched gas. The
flattening of metallicity gradients in high-mass galaxies, and the broad
scatter of the relationship are attributed to merger events. The high
metallicities and supersolar abundances shown by these galaxies imply a rapid,
high efficiency star formation. The observed [Z/H]--mass and [alpha/Fe]--mass
relationships can be interpreted as a natural outcome of an early star-forming
collapse. However, we find that hierarchical galaxy formation models
implementing mass-dependent star formation efficiency, varying IMF, energy
feedback via AGN, and the effects due to merger-induced starbursts can also
reproduce both our observed relationships.
