Optical and Near‐Infrared Structural Properties of Cluster Galaxies atz∼ 0.3

Abstract

Structural parameters (half-light radius re, mean effective surface brightness μe, and Sersic index n, parameterizing the light profile shape) are derived for a sample of galaxies in the rich cluster AC 118 at z = 0.31, so far the largest (N = 93) sample of galaxies at intermediate redshift with structural parameters measured in the near-infrared. The parameters are obtained in two optical wavebands (V and R) and in the K band, corresponding approximately to the B, V, and H rest frame. The distributions of re at z = 0.31 match those for the Coma Cluster (i.e., for the local universe) both in the optical and in the NIR. The K-band distribution is of particular interest, since the NIR light mimics the mass distribution of galaxies. The similarity of the distributions for the two clusters (AC 118 and Coma) proves that the galaxies at the bright end of the luminosity function did not significantly change their sizes since z ~ 0.3 to the present epoch. The ratio of the optical to the NIR half-light radius shows a marked trend with the shape of the light profile (Sersic index n). In galaxies with n 4 (typical bright ellipticals) re,NIR ~ 0.6re,opt, while the average ratio is 0.8 for galaxies with lower n (typical disk systems). Moreover, the NIR Sersic index is systematically larger than in the optical for n 4. These results, translated into optical and optical-NIR color gradients, imply that the optical color gradients at z ~ 0.3 are similar to those of nearby galaxies. The optical-NIR color gradients are in the average larger, ranging from -0.73 mag dex-1 for n 4 to -0.35 mag dex-1 for n 4. Models with pure or pure gradients are unable to reconcile our color gradients estimates with observations at z ~ 0, but we argue that the combined effects of age and metallicity might explain consistently the observed data: passive evolution (plus the possible effect of dust absorption) may account for the differences between the optical and NIR structural properties. The lack of any major change in re,NIR since z ~ 0.3 suggests that merging involving bright galaxies did not play a significant role in the last ~4.4 Gyr (ΩM = 0.3, ΩΛ = 0, H0 = 50 km s-1 Mpc-1). The results of the present paper will be applied to the study of the scaling laws in subsequent works.