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

Oct 4, 2026

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Detailed weak-lensing and X-ray analysis targets the Frontier Fields galaxy cluster Abell 370, using Subaru/Sprime-Cam wide-field data and Chandra X-ray observations. Free-form lensing reconstruction combines two-dimensional shear with azimuthally averaged magnification constraints to recover radial structure and 2D mass morphology. Triaxial modeling with a Navarro–Frenk–White profile constrains halo mass, concentration, and axis ratios under uninformative priors. Including a line-of-sight alignment prior favors a more prolate geometry. Results are compared to hydrostatic mass estimates, implying the likely maximum hydrostatic bias in disturbed clusters.

LanguageEnglish

CategoryResearch & Report

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Pages34

Reading Time1 h 26 min

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Abstract

Keywords

1. Introduction

Galaxy clusters as cosmological probes

Triaxial halo shapes and orientation effects

Superlens clusters and line-of-sight alignment

Case study: Abell 370

arXiv :2203 .03647v2 [ astro-ph .CO] 19 Jun 2022 DRAFT VERSION WEDNESDAY 22ND JUNE, 2022 Typeset using LATEX twocolumn style in AASTeX631 Line-of-sight Elongation and Hydrostatic Mass Bias of the Frontier Fields Galaxy Cluster Abell 370 KEIICHI UMETSU , 1 SHUTARO UEDA , 1 BAU-CHING HSIEH , 1 MARIO NONINO ,2 I-NON CHIU ,3, 4, 1 MASAMUNE OGURI ,5, 6 SANDOR M. MOLNAR , 1 ANTON M. KOEKEMOER ,7 AND SUT-IENG TAM 1 1 Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan 2INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143, Trieste, Italy 3 Tsung-Dao Lee Institute, and Key Laboratory for Particle Physics, Astrophysics and Cosmology, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China 4 Department of Astronomy, School of Physics and Astronomy, and Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai Jiao Tong University, Shanghai 200240, China 5 Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan 6 Department of Physics, Graduate School of Science, Chiba University, 1 -33 Yayoi-Cho, Inage-Ku, Chiba 263-8522, Japan 7 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA ABSTRACT 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 BRC z 0 Subaru/Sprime-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 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 M200 = (1:54 􀀆 0:29) 􀀂 1015 h 􀀀1 M􀀌 , a halo concentration c200 = 5:27 􀀆 1:28, and a minor–major axis ratio qa = 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 fgas = (8:4 􀀆 1:0)% at 0:7 h􀀀1 Mpc 􀀘 0:7r500. When compared to the hydrostatic mass estimate (MHE ) from Chandra observations, our triaxial weak-lensing analysis yields spherically enclosed mass ratios of 1 􀀀 b 􀀑 MHE =MWL = 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. Keywords: cosmology: observations—dark matter—gravitational lensing: weak—X-rays: galaxies: clusters —galaxies: clusters: individual (A370) 1. INTRODUCTION Galaxy clusters can provide a range of valuable information from the physics driving structure formation to the nature of dark matter and dark energy. Their matter content reﬂects that of the universe: 􀀘 85% dark matter and 􀀘 15% baryons, with 􀀘 90% of the baryons residing in the hot intracluster medium (ICM) . Determining the evolution of the abundance of rare massive clusters provides powerful cosmo Corresponding author: Keiichi Umetsu [keiichi@asiaa.sinica.edu.tw](keiichi@asiaa.sinica.edu.tw) logical constraints, especially on the matter density parameter, 􀀊 m , and the amplitude of linear density ﬂuctuations, 􀀛8 (e.g., see Mantz et al. 2015, and references therein) . Conversely, an accurate determination of the total mass of galaxy clusters using direct mass probes, such as weak gravitational lensing, is ess

FAQs

What observations are combined to study Abell 370 in this work?

The study combines wide-field Subaru/Sprime-Cam data with azimuthally averaged magnification constraints and Chandra X-ray observations, enabling joint weak-lensing and X-ray analysis.

How is the cluster mass distribution reconstructed?

A free-form lensing mass reconstruction uses both 2D shear and azimuthally averaged magnification constraints to determine radial structure and 2D morphology.

What is the main conclusion from comparing triaxial weak-lensing masses with hydrostatic masses?

In a highly disturbed dynamical state, the triaxial weak-lensing analysis yields mass ratios relative to hydrostatic estimates, indicating the likely maximum level of hydrostatic bias in galaxy clusters.

Oct 4, 2026

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