SLICE: Combining strong lensing and X-ray data of AC 114

Further insights into the merger scenario^★

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
² Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
³ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁴ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁵ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, CH-1290 Versoix, Switzerland
⁶ STAR Institute, Quartier Agora - Allée du six Août, 19c B-4000 Liège, Belgium
⁷ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁸ Department of Astronomy, University of Washington, Physics-Astronomy Building, Box 351580 Seattle, WA 98195-1700, USA
⁹ Astrophysics Research Centre, University of KwaZulu-Natal, Westville Campus, Durban 4041, South Africa
¹⁰ School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Westville Campus, Durban 4041, South Africa
¹¹ Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
¹² Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
¹³ DIFA – Università di Bologna, Via P. Gobetti 93/2, I-40129 Bologna, Italy
¹⁴ INAF – IASF Milano, via A. Corti 12, 20133 Milano, Italy
¹⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
¹⁶ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 December 2025
Accepted: 14 April 2026

Abstract

AC 114 is a historically significant galaxy cluster, being one of the first strong lensing clusters detected from the ground in the early 1990s, prior to the launch of the Hubble Space Telescope (HST). Despite this early prominence, no detailed lensing analyses were carried out for more than 15 years. We studied this cluster using James Webb Space Telescope (JWST) imaging obtained as part of the Strong LensIng and Cluster Evolution (SLICE) programme, complemented by archival HST and X-ray observations. JWST data reveal ten new multiply imaged systems and enabled the identification of conjugate substructures in several of the 16 systems, significantly increasing the number of strong lensing constraints. Using these data, we constructed a parametric mass model with LENSTOOL and extended it by explicitly incorporating the Chandra data in a combined strong lensing+X-ray fit, following the methodology recently introduced by us. Our best-fit model reproduces the multiple images with an RMS of 0.4″ while simultaneously matching the X-ray data. The dark matter distribution is unimodal and centred on the brightest cluster galaxy, with a large core radius of 83 ± 5 kpc, consistent with values reported in other strong lensing clusters. The strong lensing constraints require the inclusion of an external shear component whose position angle points unambiguously towards a nearby (∼1 Mpc), well-defined mass concentration at the same redshift in the north-west, for which we propose the name AC 114b. The spatial coverage of the XMM-Newton data encompasses the whole structure, allowing us to probe the X-ray properties of the companion cluster and the thermodynamics of AC 114. This provided further evidence of a major merger, in line with previous signatures seen in Chandra, radio, and optical spectroscopic data. Our results shed new light on the merging scenario, revealing a major merger caught in a late post-collisional phase, where AC 114 is the dominant system and AC 114 b has likely been stripped of its hot gas. Our analysis highlights the power of combining strong lensing constraints with X-ray data to disentangle the dark matter and gas components and to investigate the dynamical processes driving cluster mergers. Our lens model and associated products are available for download at the Strong Lensing Cluster Atlas Data Base, which is hosted at Laboratoire d’Astrophysique de Marseille.