IDCS J1426+3508: Discovery of a Massive, IR-Selected Galaxy Cluster at z
  = 1.75
  We report the discovery of an IR-selected massive galaxy cluster in the IRAC
Distant Cluster Survey (IDCS). We present new data from the Hubble Space
Telescope and the W. M. Keck Observatory that spectroscopically confirm IDCS
J1426+3508 at z=1.75. Moreover, the cluster is detected in archival Chandra
data as an extended X-ray source, comprising 54 counts after the removal of
point sources. We calculate an X-ray luminosity of L{0.5-2 keV} = (5.5 +/- 1.2)
X 1e44 ergs/s within r = 60 arcsec (~1 Mpc diameter), which implies M_{200,L_x}
= (5.6 +/- 1.6) X 1e14 Msun. IDCS J1426+3508 appears to be an exceptionally
massive cluster for its redshift.
