The XMM Cluster Survey: The build up of stellar mass in Brightest
  Cluster Galaxies at high redshift
  We present deep J and Ks band photometry of 20 high redshift galaxy clusters
between z=0.8-1.5, 19 of which are observed with the MOIRCS instrument on the
Subaru Telescope. By using near-infrared light as a proxy for stellar mass we
find the surprising result that the average stellar mass of Brightest Cluster
Galaxies (BCGs) has remained constant at ~9e11MSol since z~1.5. We investigate
the effect on this result of differing star formation histories generated by
three well known and independent stellar population codes and find it to be
robust for reasonable, physically motivated choices of age and metallicity. By
performing Monte Carlo simulations we find that the result is unaffected by any
correlation between BCG mass and cluster mass in either the observed or model
clusters. The large stellar masses imply that the assemblage of these galaxies
took place at the same time as the initial burst of star formation. This result
leads us to conclude that dry merging has had little effect on the average
stellar mass of BCGs over the last 9-10 Gyr in stark contrast to the
predictions of semi-analytic models, based on the hierarchical merging of dark
matter haloes, which predict a more protracted mass build up over a Hubble
time. We discuss however that there is potential for reconciliation between
observation and theory if there is a significant growth of material in the
intracluster light over the same period.
