On the Radial Stellar Content of Early-Type Galaxies as a Function of
  Mass and Environment
  Using optical-optical and optical-NIR colors, we analyze the radial
dependence of age and metallicity inside massive (M* > 10^10.5 MSun),
low-redshift (z<0.1), early-type galaxies (ETGs), residing in both high-density
group regions and the field. On average, internal color gradients of ETGs are
mainly driven by metallicity, consistent with previous studies. However, we
find that group galaxies feature positive age gradients, Nabla_t, i.e. a
younger stellar population in the galaxy center, and steeper metallicity
gradients, compared to the field sample, whose Nabla_t ranges from negative in
lower mass galaxies, to positive gradients at higher mass. These dependencies
yield new constraints to models of galaxy formation and evolution. We speculate
that age and metallicity gradients of group ETGs result from (either gas-rich
or minor-dry) mergers and/or cold-gas accretion, while field ETGs exhibit the
characteristic flatter gradients expected from younger, more metal-rich, stars
formed inside--out by later gas-cooling.
