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 (RELICS). Using Nod & Shuffle multi-slit observations with LDSS-3 on Magellan, we identify ${\sim}50$ cluster members and derive a cluster redshift of $z_c = 0.972$, with a velocity dispersion of $\sigma = 1235 \pm 170\ \textrm{km}\ \textrm{s}^{-1}$. We calculate a cluster mass using a $\sigma_{200}-M_{200}$ scaling relation of $M_{200} = (9.4 \pm 3.6) \times 10^{14}\ M_\odot$, 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.