The evolution of the color gradients of early-type cluster galaxies

We investigate the origin of color gradients in cluster early-type galaxies to probe whether pure age or pure metallicity gradients can explain the ob served data in local and distant(z approximate to 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 ta ken 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 d ecade. 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 approximate to 0.4. We analyse the results with up to date stellar population models. We find that passive evolution of metallicity gr adients (approximate to 0.2 dex per radial decade) provides a consistent ex planation of the local and distant galaxies' data. Invoking pure age gradie nts (with fixed metallicity) to explain local color gradients produces too steep gradients at redshifts z approximate to 0.4. Pure age gradients are c onsistent with the data only if large present day ages greater than or equa l to 15 Gyr are assumed for the galaxy centers.