HST Grism Observations of a z~1.8 Cluster Candidate from the Clusters Occupied by Bent Radio AGN (COBRA) Survey

Abstract

We present new Hubble Space Telescope/Wide Field Camera 3 G141 grism observations for COBRA1411+3415, 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 $1.8006\le z_{grism}\le 1.8175$, consistent with COBRA1411+3415 being a high-redshift cluster with a mean redshift of $⟨z_{grism}⟩=1.8106\pm 0.0006$. 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 ${\sigma }_{\parallel }=701_{-138}^{+347}$ km/s with a virial mass of $M_{200}\approx 2.2_{-1.3}^{+3.3}\times 10^{14}$ M${}_{\odot }$. However, the mass may be lower if the cluster is still in formation. In projected phase-space, we also identify two possible infalling members of COBRA1411+3415 and two additional structures at $z\sim 1.73$ and $z\sim 1.88$. The similar spatial distributions and small projected separation from the main cluster suggest they could be a part of the same large-scale filament and together may form a protocluster system that could eventually merge to form a single, massive cluster. COBRA1411+3415 is the highest redshift cluster to be spectroscopically confirmed using a bent, double-lobed radio source as a cluster tracer.

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