An IR-Selected Galaxy Cluster at z=1.27

Abstract

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 (0.49h -1 100 Mpc, q 0 = 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 σ = 700 ± 180 km s -1 , which is similar to that of Abell richness class R = 1 clusters in the present epoch. A deep ROSAT/PSPC observation detects X-ray emission at the 4.5σ 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 are 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 × 10 44 ergs s -1 suggests the presence of a moderately massive system. ClG J0848 + 4453 is the highest redshift cluster found in a field survey. © 1997 American Astronomical Society.