Line-of-sight Elongation and Hydrostatic Mass Bias of the Frontier Fields Galaxy Cluster Abell 370

We present a detailed weak-lensing and X-ray study of the Frontier Fields galaxy cluster Abell 370, one of the most massive known lenses on the sky, using wide-field BR (C) z ' Subaru/Suprime-Cam and Chandra X-ray observations. By combining two-dimensional (2D) shear and azimuthally averaged magnification constraints derived from Subaru data, we perform a lensing mass reconstruction in a free-form manner, which allows us to determine both the radial structure and 2D morphology of the cluster mass distribution. In a triaxial framework assuming a Navarro-Frenk-White density profile, we constrain the intrinsic structure and geometry of the cluster halo by forward modeling the reconstructed mass map. We obtain a halo mass M (200) = (1.54 +/- 0.29) x10(15) h (-1) M (circle dot), a halo concentration c (200) = 5.27 +/- 1.28, and a minor-major axis ratio q ( a ) = 0.62 +/- 0.23 with uninformative priors. Using a prior on the line-of-sight alignment of the halo major axis derived from binary merger simulations constrained by multi-probe observations, we find that the data favor a more prolate geometry with lower mass and lower concentration. From triaxial lens modeling with the line-of-sight prior, we find a spherically enclosed gas mass fraction of f (gas) = (8.4 +/- 1.0)% at 0.7 h (-1) Mpc similar to 0.7r (500). When compared to the hydrostatic mass estimate (M (HE)) from Chandra observations, our triaxial weak-lensing analysis yields spherically enclosed mass ratios of 1 - b equivalent to M (HE)/M (WL) = 0.56 +/- 0.09 and 0.51 +/- 0.09 at 0.7 h (-1) Mpc with and without using the line-of-sight prior, respectively. Since the cluster is in a highly disturbed dynamical state, this represents the likely maximum level of hydrostatic bias in galaxy clusters.