HST Observations used in analysis of z=1.8 COBRA Cluster

We present new HST/WFC3 grism observations for \targ, originally identified as a high-redshift cluster candidate in the Clusters Occupied by Bent Radio AGN (COBRA) survey using radio, infrared, and optical data. We spectroscopically identify seven cluster members within a 0.5 Mpc radius with grism redshifts in the range $1.8006 \leq z_{grism} \leq 1.8175$, consistent with \targ\ being a high-redshift cluster with a mean redshift of $\langle z_{grism}\rangle = 1.8106 \pm 0.0006$. The detection of seven galaxies within this small redshift range is significant above the background distribution of galaxies at the level of 5$\sigma$. The line-of-sight velocity dispersion of the cluster is found to be $\sigma_{\parallel} =$ \fe{701}{347}{138} km/s with a virial mass of $M_{200} \approx$ \fe{4.1}{5.9}{2.4}$\times 10^{14}$ M$_{\odot}$. However, the mass may be lower if the cluster is still in formation. \targ\ is the highest redshift cluster to be spectroscopically confirmed using a bent, double-lobed radio source as a cluster tracer.