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

Structural parameters (half-light radius $r_e$, mean effective surfacebrightness $<\mu>_e$, and Sersic index n) are derived for a sample of galaxiesin the rich cluster AC 118 at z=0.31: so far the largest (N=93) sample ofgalaxies at intermediate-redshift with structural parameters measured in thenear-infrared. The parameters are obtained in two optical wavebands (V and R)and in the K-band. The distributions of $r_e$ at z=0.31 match those for theComa cluster both in the optical and in the NIR. The similarity of the twodistributions proves that the galaxies at the bright end of the luminosityfunction did not significantly change their sizes since $z\sim0.3$ to thepresent epoch. The ratio of the optical to the NIR half-light radius shows amarked trend with the Sersic index n. In galaxies with $n\gtrsim4$ (typicalbright ellipticals) $r_{e,NIR}\sim 0.6 r_{e,opt}$, while the average ratio is0.8 for galaxies with lower n (typical disk systems). Moreover, the NIR Sersicindex is systematically larger than in the optical for $n\lesssim4$. Theseresults, translated into optical and optical-NIR color gradients, imply thatthe optical color gradients at $z\sim$0.3 are similar to those of nearbygalaxies. The optical-NIR color gradients are larger, ranging from -0.73mag/dex for $n\lesssim4$ to -0.35 mag/dex for $n\gtrsim4$. We discuss theseresults in terms of 'pure age' and 'pure metallicity' gradient models. The lackof any major change in $r_{e,NIR}$ since $z\sim0.3$ suggests that merginginvolving bright galaxies did not play a significant role in the last $\sim$4.4Gyr.Comment: 33 pages, 12 figures, 8 tables. ApJ in pres