  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.
