The extreme synchronicity of stellar ages of red galaxies in the JKCS041
  cluster at z=2.2
  Above redshift z ~ 1.4, we known little or nothing about the stellar ages of
red galaxies in clusters, yet at these high redshifts important changes are
predicted by current renditions of galaxy formation models embedded in the
standard hierarchical paradigm of structure formation. Red-sequence galaxies in
the cluster JKCS041 at z=2.2 show a tight distribution in colour that indicates
a star formation history that is highly synchronized across galaxies.
Specifically, we measure a spread in stellar age of 160+/-30 Myr, in marked
disagreement with the current understanding of how massive red galaxies form in
clusters, i.e. if they are produced somewhat stochastically in merging episodes
that sometimes involve gas, hence star formation. The existence of a tight
distribution in colour when the universe was at one quarter of its current age
implies that mechanisms that have not yet been implemented in current galaxy
formation scenarios long ago began to shape the star formation history of red
cluster galaxies.
