Color gradients in early-type galaxies in clusters at redshift 0.37-0.56

Color gradients in elliptical galaxies in distant clusters (z = 0.37-0.56) are examined by using the archival deep imaging data of the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST). Obtained color gradients are compared with the two model gradients to examine the origin of the color gradients. In one model, a color gradient is assumed to be caused by a metallicity gradient of stellar populations, while in the other one, it is caused by an age gradient. Both of these model color gradients reproduce the average color gradient seen in nearby elliptical galaxies, but predict significantly different gradients at redshift larger than similar to 0.3. Comparison of the observed gradients and the model gradients favors the metallicity gradient much more than the age gradient as the primary origin of color gradients in elliptical galaxies in clusters. The same conclusion has been obtained for field elliptical galaxies by using those at redshift 0.1-1.0 in the Hubble Deep Field North by Tamura et al. Thus, it is also suggested that the primary origin of the color gradients in elliptical galaxies does not depend on galaxy environment.