SPIDER. IV. OPTICAL AND NEAR-INFRARED COLOR GRADIENTS IN EARLY-TYPE GALAXIES: NEW INSIGHT INTO CORRELATIONS WITH GALAXY PROPERTIES

We present an analysis of stellar population gradients in 4546 early-type galaxies (ETGs) with photometry in grizYHJK along with optical spectroscopy. ETGs were selected as bulge-dominated systems, displaying passive spectra within the SDSS fibers. A new approach is described which utilizes color information to constrain age and metallicity gradients. Defining an effective color gradient, del(star), which incorporates all of the available color indices, we investigate how del(star) varies with galaxy mass proxies, i.e., velocity dispersion, stellar (M(star)) and dynamical (M(dyn)) masses, as well as age, metallicity, and [alpha/Fe]. ETGs with M(dyn) larger than 8.5 x 10(10) M(circle dot) have increasing age gradients and decreasing metallicity gradients with respect to mass, metallicity, and enhancement. We find that velocity dispersion and [alpha/Fe] are the main drivers of these correlations. ETGs with 2.5 x 10(10) M(circle dot) <= M(dyn) <= 8.5 x 10(10) M(circle dot) show no correlation of age, metallicity, and color gradients with respect to mass, although color gradients still correlate with stellar population parameters, and these correlations are independent of each other. In both mass regimes, the striking anti-correlation between color gradient and alpha-enhancement is significant at similar to 5 sigma and results from the fact that metallicity gradient decreases with [alpha/Fe]. This anti-correlation may reflect the fact that star formation and metallicity enrichment are regulated by the interplay between the energy input from supernovae, and the temperature and pressure of the hot X-ray gas in ETGs. For all mass ranges, positive age gradients are associated with old galaxies (>5-7 Gyr). For galaxies younger than similar to 5 Gyr, mostly at low mass, the age gradient tends to be anti-correlated with the Age parameter, with more positive gradients at younger ages.