2000A&A...360..911S

We investigate the origin of color gradients in cluster early-type galaxies to probe whether pure age or pure metallicity gradients can explain the observed data in local and distant (z≃0.4) samples. We measure the surface brightness profiles of the 20 brightest early-type galaxies of CL0949+44 (hereafter CL0949) at redshift z=0.35-0.38 from HST WF2 frames taken in the filters F555W, F675W, F814W. We determine the color profiles (V-R)(r), (V-I)(r), and (R-I)(r) as a function of the radial distance r in arcsec, and fit logarithmic gradients in the range -0.2 to 0.1 mag per decade. These values are similar to what is found locally for the colors (U-B), (U-V), (B-V) which approximately match the (V-R), (V-I), (R-I) at redshift ≃0.4. We analyse the results with up to date stellar population models. We find that passive evolution of metallicity gradients (≃0.2dex per radial decade) provides a consistent explanation of the local and distant galaxies' data. Invoking pure age gradients (with fixed metallicity) to explain local color gradients produces too steep gradients at redshifts z≃0.4. Pure age gradients are consistent with the data only if large present day ages ≥15Gyr are assumed for the galaxy centers.