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

Abstract

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 (≈ 0.2 dex 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 >= 15 Gyr are assumed for the galaxy centers. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.