| Summary: | 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[superscript 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 [subscript 500, SZ] = 3.2 ± 0.8 × 10[superscript 14] M [subscript ☉] h [-1 over 70], corresponding to M [subscript 200, SZ] = 5.8 ± 1.4 × 10[superscript 14] M [subscript ☉] h [-1 over 70]. The velocity dispersion measured entirely from blue star-forming members is σ [subscript v] = 1500 ± 520 km s[superscript –1]. The prevalence of star-forming cluster members (galaxies with >1.5 M [subscript ☉] yr[superscript –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 [> over ~] 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.
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