  We present a multi-wavelength study of galaxy populations in the core of the
massive, X-ray luminous cluster XMMU J2235 at z=1.39, based on VLT and HST
optical and near-infrared photometry. Luminosity functions in the z, H, and Ks
bands show a faint-end slope consistent with being flat, and a characteristic
magnitude M* close to passive evolution predictions of M* of local massive
clusters, with a formation redshift z>2. The color-magnitude and color-mass
diagrams show evidence of a tight red sequence of massive galaxies, with
overall old stellar populations, generally early-type morphology, typically
showing early-type spectral features and rest-frame far-UV emission consistent
with very low star formation rates (SFR<0.2Msun/yr). Star forming spectroscopic
members, with SFRs of up to ~100Msun/yr, are all located at clustercentric
distances >~250kpc, with the central cluster region already appearing
effectively quenched. Massive galaxies in the core of this cluster appear to be
in an advanced evolutionary stage in terms of both star formation and mass
assembly. The high-mass end of the galaxy stellar mass function is essentially
already in place, and the stellar mass fraction estimated within r500 (~1%,
Kroupa IMF) is already similar to that of local massive clusters. On the other
hand, morphological analysis of the massive red sequence galaxies suggests that
they are smaller than similarly massive local early-types. While possibly
affected by systematics and biases, this result might imply that, in spite of
the overall early assembly of these sources, their evolution is not complete,
and processes like minor (and likely dry) merging might still shape their
structural properties to resemble those of their local counterparts, without
substantially affecting their stellar mass or host stellar
populations.[abridged]
