SPT-CL J2040–4451: AN SZ-SELECTED GALAXY CLUSTER ATz= 1.478 WITH SIGNIFICANT ONGOING STAR FORMATION

SPT-CL J2040−4451—spectroscopically confirmed at z = 1.478—is the highest-redshift galaxy cluster yet discovered via the Sunyaev–Zel'dovich effect. SPT-CL J2040−4451 was a candidate galaxy cluster identified in the first 720 deg 2 of the South Pole Telescope Sunyaev–Zel'dovich (SPT-SZ) survey, and has been confirmed in follow-up imaging and spectroscopy. From multi-object spectroscopy with Magellan-I/Baade+IMACS we measure spectroscopic redshifts for 15 cluster member galaxies, all of which have strong [O ii ] λλ3727 emission. SPT-CL J2040−4451 has an SZ-measured mass of M 500, SZ = 3.2 ± 0.8 × 10 14 M ☉ h , corresponding to M 200, SZ = 5.8 ± 1.4 × 10 14 M ☉ h . The velocity dispersion measured entirely from blue star-forming members is σ v = 1500 ± 520 km s −1 . The prevalence of star-forming cluster members (galaxies with >1.5 M ☉ yr −1 ) implies that this massive, high-redshift cluster is experiencing a phase of active star formation, and supports recent results showing a marked increase in star formation occurring in galaxy clusters at z ≳ 1.4. We also compute the probability of finding a cluster as rare as this in the SPT-SZ survey to be >99%, indicating that its discovery is not in tension with the concordance ΛCDM cosmological model.