HST Grism Observations of a <i>z</i> ∼ 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 COBRA 1411+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 ≤ z grism ≤ 1.8175, consistent with COBRA 1411+3415 being a high-redshift cluster with a mean redshift of 〈 z grism 〉 = 1.8106 ± 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 σ . The line-of-sight velocity dispersion of the cluster is found to be σ ∥ = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mn>701</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>138</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>347</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km s −1 with a virial mass of M 200 ≈ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3</mml:mn> <mml:mo>.</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> × 10 14 M ⊙ . 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 COBRA 1411+3415 and two additional structures at z ∼ 1.73 and z ∼ 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. COBRA 1411+3415 is the highest-redshift cluster to be spectroscopically confirmed using a bent, double-lobed radio source as a cluster tracer.