An X-ray Selected Galaxy Cluster in the Lockman Hole at Redshift 1.753
  We have discovered an X-ray selected galaxy cluster with a spectroscopic
redshift of 1.753. The redshift is of the brightest cluster galaxy (BCG), which
is coincident with the peak of the X-ray surface brightness. We also have
concordant photometric redshifts for seven additional candidate cluster
members. The X-ray luminosity of the cluster is 3.68 +/- 0.70 x 10^43 erg s^-1
in the 0.1 - 2.4 keV band. The optical/IR properties of the BCG imply its
formation redshift was ~5 if its stars formed in a short burst. This result
continues the trend from lower redshift in which the observed properties of
BCGs are most simply explained by a monolithic collapse at very high redshift
instead of the theoretically preferred gradual hierarchical assembly at later
times. However the models corresponding to different formation redshifts are
more clearly separated as our observation epoch approaches the galaxy formation
epoch. Although our infrared photometry is not deep enough to define a red
sequence, we do identify a few galaxies at the cluster redshift that have the
expected red sequence photometric properties.
