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(star) greater than or similar to 10(10.5) M(circle dot)), 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, del(t), i.e., a younger stellar population in the galaxy center, and steeper metallicity gradients, compared to the field sample, whose del(t) ranges from negative in lower mass galaxies to positive gradients at higher mass. These dependencies yield new constraints on 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.