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

Abstract

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) × 1014 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.