  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 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 ~ 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.
