IDCS J1426.5+3508: The Most Massive Galaxy Cluster at $z > 1.5$
  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 M$_{500}$ = 2.3-3.3 $\times 10^{14}$
M$_{\odot}$, with the low-scatter $Y_X$-based mass $M_{500,Y_X} =
2.6^{+1.5}_{-0.5} \times 10^{14}$ M$_\odot$. 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 2$\sigma$ upper limit of $Z(r < R_{500}) < 0.18
Z_{\odot}$. 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 $\sim$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
($\lesssim$500 Myr) merger/interaction with another massive system.
