We use HAWK-I , the recently-commissioned near-IR imager on Yepun ( VLT-UT4 ) , to obtain wide-field , high-resolution images of the X-ray luminous galaxy cluster XMMU J2235.3-2557 in the J and Ks bands , and we use these images to build a colour-magnitude diagram of cluster galaxies .
Galaxies in the core of the cluster form a tight red sequence with a mean \mathrm { J - Ks } colour of 1.9 ( Vega system ) .
The intrinsic scatter in the colour of galaxies that lie on the red sequence is similar to that measured for galaxies on the red sequence of the Coma cluster .
The slope and location of the red sequence can be modelled by passively evolving the red sequence of the Coma cluster backwards in time .
Using simple stellar population ( SSP ) models , we find that galaxies in the core of XMMU J2235.3-2557 are , even at z = 1.39 , already 3 Gyr old , corresponding to a formation redshift of z _ { f } \sim 4 .
Outside the core , the intrinsic scatter and the fraction of galaxies actively forming stars increase substantially .
Using SSP models , we find that most of these galaxies will join the red sequence within 1.5 Gyr .
The contrast between galaxies in the cluster core and galaxies in the cluster outskirts indicates that the red sequence of XMMU J2235.3-2557 is being built from the dense cluster core outwards .