Assembling a RELIC at Redshift 1: Spectroscopic Observations of Galaxies in the RELICS Cluster SPT-CLJ0615−5746

Abstract

We present a catalog of spectroscopic redshifts for SPT-CLJ0615−5746, the most distant cluster in the Reionization Lensing Cluster Survey. Using Nod & Shuffle multislit observations with LDSS-3 on Magellan, we identify ∼50 cluster members and derive a cluster redshift of z c  = 0.972, with a velocity dispersion of σ  = 1244 ± 162 km s −1 . We calculate a cluster mass using a σ 200  −  M 200 scaling relation of M 200  = (9.6 ± 3.5) × 10 14 M ⊙ , in agreement with previous, independent mass measurements of this cluster. In addition, we examine the kinematic state of SPT-CLJ0615−5746, taking into consideration prior investigations of this system. With an elongated profile in lensing mass and X-ray emission, a non-Gaussian velocity dispersion that increases with clustercentric radius, and a brightest cluster galaxy not at rest with the bulk of the system, there are multiple cluster properties that, while not individually compelling, combine to paint a picture that SPT-CLJ0615−5746 is currently being assembled.