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We report the detection of color gradients in six massive (stellar mass >10^{10} M_{sun}) and passively evolving (specific SFR < 10^{-11}/yr) galaxiesat redshift 1.3<z<2.5 identified in the HUDF using HST ACS and WFC3/IR images.After matching different PSFs, we obtain color maps and multi-bandoptical/near-IR photometry (BVizYJH) in concentric annuli, from the smallestresolved radial (~1.7 kpc) up to several times the H-band effective radius. Wefind that the inner regions of these galaxies have redder rest-frameUV--optical colors than the outer parts. The slopes of the color gradientsmildly depend on the overall dust obscuration and rest-frame (U-V) color, withmore obscured or redder galaxies having steeper color gradients. The z~2 colorgradients are also steeper than those of local early-types. The gradient of asingle parameter (age, extinction or metallicity) cannot fully explain theobserved color gradients. Fitting spatially resolved HST seven-band photometryto stellar population synthesis models, we find that, regardless of assumptionsfor metallicity gradient, the redder inner regions of the galaxies haveslightly higher dust obscuration than the bluer outer regions, although themagnitude depends on the assumed extinction law. The derived age gradientdepends on the assumptions for metallicity gradient. We discuss theimplications of a number of assumptions for metallicity gradient on theformation and evolution of these galaxies. We find that the evolution of themass--size relationship from z~2 to z~0 cannot be driven by in--situ extendedstar formation, implying that accretion or merger is mostly responsible for theevolution. The lack of a correlation between color gradient and stellar massargues against the metallicity gradient predicted by the monolithic collapse,which would require significant major mergers to evolve into the one observedat z~0. (Abridged)

Color and Stellar Population Gradients in Passively Evolving Galaxies at z~2 from HST/WFC3 Deep Imaging in the Hubble Ultra Deep Field