We study the structure and the internal colour gradients of cluster galaxies from UV to NIR restframe , in the redshift range \mathrm { z = 0.21 - 0.64 } .
Structural parameters ( half light radius \mathrm { r _ { e } } , mean surface brightness \mathrm { < \mu > _ { e } } and Sersic index \mathrm { n } ) are derived for galaxies in the clusters \mathrm { A 209 } at \mathrm { z = 0.21 } and \mathrm { EIS 0048 } at \mathrm { z = 0.64 } .
This data set , together with previous data for the cluster \mathrm { AC 118 } at \mathrm { z = 0.31 } , constitutes the first large ( \mathrm { N \sim 270 } ) sample of cluster galaxies whose internal structure in UV , optical ( OPT ) and NIR ( U- , V- and H-band restframe ) can be investigated up to a look-back time of \mathrm { \sim 6 ~ { } Gyr } ( \Omega _ { m } = 0.3 , \Omega _ { \Lambda } = 0.7 and \mathrm { H _ { 0 } = 70 ~ { } Km~ { } s ^ { -1 } Mpc ^ { -1 } } ) .
Galaxies are classified as spheroids or disks according to the shape of the light profile , and the evolution of the two populations are investigated separately .
On average , both spheroids and disks are more concentrated at longer wavelengths : the galaxy sizes become smaller from UV to NIR , while Sersic indices increase .
This trend shows an evolution in disks , where the mean ratio of optical to NIR Sersic indices decreases from \mathrm { z = 0.31 } to \mathrm { z = 0.64 } .
Colour gradients are on average negative at all redshifts and are stronger in disks than in spheroids .
But while for spheroids both \mathrm { grad ( UV - OPT ) } and \mathrm { grad ( OPT - NIR ) } are only weakly dependent on redshift , the optical-NIR gradients of disks become significantly smaller at \mathrm { z = 0.64 } .
Colour gradients and central colours are compared with models of metallicity , age , and dust extinction gradients .
Metallicity turns out to be the primary driver of colour gradients in spheroids , the age gradient being constrained to be smaller than \sim 25 \% .
For disks , two kinds of models fit the present data : ( i ) age gradients ( in the range \left [ 30 , 50 \right ] \% ) with significant dust extinction , and ( ii ) ‘ pure ’ dust models , in which the gradients of colour excess are a factor of two higher in \mathrm { EIS 0048 } than in the other clusters .
Since colour gradients of disks seem not to correlate significantly with inclination , we argue that age gradient models could represent a more likely explanation of the present data , in agreement with what expected on the basis of hierarchical merging scenarios .