IDCS J1433.2+3306: An IR-Selected Galaxy Cluster at z = 1.89

Abstract: We report the discovery of an IR-selected galaxy cluster in the IRAC Distant Cluster Survey (IDCS). New data from the Hubble Space Telescope spectroscopically confirm IDCS J1433.2+3306 at z = 1.89 with robust spectroscopic redshifts for seven members, two of which are based on the 4000 Angstrom break. Detected emission lines such as [OII] and Hbeta indicate star formation rates of >20 solar masses per year for three galaxies within a 500 kpc projected radius of the cluster center. The cluster exhibits a red sequence with a scatter and color indicative of a formation redshift z > 3.5. The stellar age of the early-type galaxy population is approximately consistent with those of clusters at lower redshift (1 < z < 1.5) suggesting that clusters at these redshifts are experiencing ongoing or increasing star formation.