An IR-Selected Galaxy Cluster at z=1.27

We report the discovery of a galaxy cluster at z=1.27. ClG J0848+4453 was found in a near-IR field survey as a high density region of objects with very red J-K colors. Optical spectroscopy of a limited number of 24 < R < 25 objects in the area shows that 6 galaxies within a 90 arcsec (0.49/h Mpc, q_O = 0.1) diameter region lie at z=1.273 +/- 0.002. Most of these 6 member galaxies have broad-band colors consistent with the expected spectral energy distribution of a passively-evolving elliptical galaxy formed at high redshift. An additional 2 galaxies located ~2 arcmin from the cluster center are also at z=1.27. Using all 8 of these spectroscopic members, we estimate the velocity dispersion is 700 +/- 180 km/s, similar to that of Abell R=1 clusters in the present epoch. A deep Rosat PSPC observation detects X-ray emission at the 5 sigma level coincident with the nominal cluster center. Assuming that the X-ray flux is emitted by hot gas trapped in the potential well of a collapsed system (no AGN is known to exist in the area), the resulting X-ray luminosity in the rest frame 0.1-2.4 keV band of L_x = 1.5 x 10^44 ergs/s suggests the presence of a moderately large mass. ClG J0848+4453 is the highest redshift cluster found without targetting a central active galaxy.