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[subscript X]–M, f[subscript g]–M, Y[subscript X]–M, and L[subscript X]–M, finding a tight distribution of masses from these different methods, spanning M[subscript 500] = 2.3–3.3 × 10[superscript 14]M[subscript ⊙], with the low-scatter Y[subscript X]-based mass M[subscript 500,Y[subscript X]] = 2.6[+1.5 over -0.5] x 10[superscript 14] M[subscript ⊙]. 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[subscript 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 < R[subscript 500]) < 0.18 Z[subscript ⊙]. 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 ([< over ~]500 Myr) merger/interaction with another massive system.