Structural parameters ( half-light radius r _ { e } , mean effective surface brightness \langle \mu \rangle _ { e } , 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 B , V and H rest-frame .

The distributions of r _ { e } 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 \sim 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 \gtrsim 4 ( typical bright ellipticals ) r _ { e,NIR } \sim 0.6 r _ { e,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 \lesssim 4 .
These results , translated into optical and optical-NIR color gradients , imply that the optical color gradients at z \sim 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 for n \lesssim 4 to -0.35 mag/dex for n \gtrsim 4 .

Models with ‘ pure age ’ or ‘ pure metallicity ’ gradients are unable to reconcile our color gradients estimates with observations at z \sim 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 r _ { e,NIR } since z \sim 0.3 suggests that merging involving bright galaxies did not play a significant role in the last \sim 4.4 Gyr ( \Omega _ { M } =0.3 , \Omega _ { \Lambda } = 0 , H _ { 0 } = 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 .