Spatially Resolved Colors and Stellar Population Properties in Early-Type Galaxies at z ~ 1.5

Abstract

We present F850LP-F160W color gradients for 11 early-type galaxies (ETGs) at 1.0<z_spec<1.9. Significant negative F850LP-F160W color gradients have been detected in ~70% of our sample within the effective radius R_e, the remaining 30% having a flat color profile. Extending the analysis at R>R_e we have found that the fraction of high-z ETGs with negative F850LP-F160W color gradients rises up to 100%. For each galaxy, we investigate the origin of the radial color variation with a technique based on the matching of both the spatially resolved color and the global spectral energy distribution (SED) to predictions of composite stellar population models. In fact, we find that the age of the stellar populations is the only parameter whose radial variation alone can fully account for the observed color gradients and global SEDs for half of the galaxies in our sample (6 ETGs), without the need of radial variation of any other stellar population property. For four out of these six ETGs, a pure metallicity variation can also reproduce the detected color gradients. Nonetheless, a minor contribution to the observed color gradients from radial variation of star-formation time scale, abundance of low-to-high mass stars and dust cannot be completely ruled out. For the remaining half of the sample, our analysis suggests a more complex scenario whereby more properties of the stellar populations need to simultaneously vary to generate the observed color gradients and global SED. Our results show that, despite the young mean age of our galaxies (<3-4 Gyr), they already exhibit significant differences among their stellar content. We have discussed our results within the framework of the widest accepted scenarios of galaxy formation and conclude that none of them can satisfactorily account for the observed distribution of color gradients and for the spatially resolved content of high-z ETGs.