Multi-wavelength observations of a rich galaxy cluster at z ~ 1: the
  HST/ACS colour-magnitude diagram
  XMMU J1229+0151 is a rich galaxy cluster with redshift z=0.975, that was
serendipitously detected in X-rays within the scope of the XMM-Newton Distant
Cluster Project. HST/ACS observations in the i775 and z850 passbands, as well
as VLT/FORS2 spectroscopy were further obtained, in addition to follow-up
Near-Infrared (NIR) imaging in J- and Ks-bands with NTT/SOFI. We investigate
the photometric, structural and spectral properties of the early-type galaxies
in the high-redshift cluster XMMU J1229+0151. Source detection and aperture
photometry are performed in the optical and NIR imaging. Galaxy morphology is
inspected visually and by means of Sersic profile fitting to the 21
spectroscopically confirmed cluster members in the ACS field of view. The
i775-z850 colour-magnitude relation (CMR) is derived with a method based on
galaxy magnitudes obtained by fitting the surface brightness of the galaxies
with Sersic models. The i775-z850 CMR of the spectroscopic members shows a very
tight red-sequence with a zero point of 0.86+-0.04 mag and intrinsic scatter
equal to 0.039 mag. The CMR obtained with the galaxy models has similar
parameters. Stellar masses and formation ages of the cluster galaxies are
derived by fitting the observed spectral energy distributions (SED) with models
based on Bruzual & Charlot 2003. We obtain a star formation weighted age of 4.3
Gyr for a median mass of 7.4e10 Msun. Instead of an unambiguous brightest
cluster galaxy (BCG), we find three bright galaxies with a similar z850
magnitude, which are, in addition, the most massive cluster members, with ~
2e11 Msun. Our results strengthen the current evidence for a lack of
significant evolution of the scatter and slope of the red-sequence out to z~1.
