IDCS J1426.5+3508: The Most Massive Galaxy Cluster at z > 1.5

Abstract

We present a deep (100 ks) Chandra observation of IDCS J1426.5+3508, a spectroscopically confirmed, infrared-selected galaxy cluster at z = 1.75. This cluster is the most massive galaxy cluster currently known at z > 1.5, based on existing Sunyaev-Zel’dovich (SZ) and gravitational lensing detections. We confirm this high mass via a variety of X-ray scaling relations, including T_X-M, f_g-M, Y_X-M, and L_X-M, finding a tight distribution of masses from these different methods, spanning M₅₀₀ = 2.3-3.3 × 10¹⁴M_⊙, with the low-scatter Y_X-based mass {M}_500,{Y{{X}}} = {2.6}_-0.5^+1.5× {10}¹⁴ M_⊙. IDCS J1426.5+3508 is currently the only cluster at z > 1.5 for which X-ray, SZ, and gravitational lensing mass estimates exist, and these are in remarkably good agreement. We find a relatively tight distribution of the gas-to-total mass ratio, employing total masses from all of the aforementioned indicators, with values ranging from f_gas,500 = 0.087-0.12. We do not detect metals in the intracluster medium (ICM) of this system, placing a 2σ upper limit of Z(r\lt {R}₅₀₀)\lt 0.18 {Z}_⊙ . This upper limit on the metallicity suggests that this system may still be in the process of enriching its ICM. The cluster has a dense, low-entropy core, offset by ∼30 kpc from the X-ray centroid, which makes it one of the few “cool core” clusters discovered at z > 1, and the first known cool core cluster at z > 1.2. The offset of this core from the large-scale centroid suggests that this cluster has had a relatively recent (≲500 Myr) merger/interaction with another massive system.