JWST lens model for A370: A very low dark matter fraction for a brightest cluster galaxy and lensing properties for the Dragon arc

¹ Instituto de Física de Cantabria (CSIC-UC), Avda. Los Castros s/n, 39005 Santander, Spain
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
³ Department of Astronomy, Tsinghua University, Beijing 100084, China
⁴ Steward Observatory, University of Arizona, 933 N Cherry Avenue, Tucson, AZ 85721, USA
⁵ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
⁶ Cosmic Dawn Center (DAWN), 2200 Copenhagen, Denmark
⁷ Univ. Grenoble Alpes, CNRS, Grenoble INP*, LPSC-IN2P3, 38000 Grenoble, France
⁸ Department of Physics, Oklahoma State University, 145 Physical Sciences Building, Stillwater, OK 74078, USA
⁹ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004, USA
¹⁰ Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
¹¹ Department of Theoretical Physics, University of Basque Country UPV/EHU, Bilbao, Spain
¹² Ikerbasque, Basque Foundation for Science, Bilbao, Spain
¹³ Donostia International Physics Center, Paseo Manuel de Lardizabal, 4, San Sebastián 20018, Spain

^⋆ Corresponding author: jdiego@ifca.unican.es

Received: 23 June 2025
Accepted: 24 September 2025

Abstract

We present a new lens model for the z = 0.375 galaxy cluster Abell 370, based on previously spectroscopically confirmed lensed galaxies and new lensed systems identified in JWST data, including recent data from the MAGNIF program. Based on the best models capable of reproducing two radial arcs near the brightest cluster galaxies (BCGs), we were able to compare the stellar mass to the total mass from the lens model. We find that the fraction of dark matter in the south BCG is consistent with ΛCDM, while in the north BCG, we find a very small amount of dark matter, which is more consistent with alternative models. We discuss possible causes for this and conclude that additional data is needed to clarify the situation. We studied the lensing properties, magnification, time delay and strength of the critical curve, along the Dragon arc, where previous studies have reported tens of alleged microlensing events from supergiant stars at z = 0.7251. The new lens model is able to reproduce the distribution of microlensing events with a very high level of accuracy. Some of the microlensing events might be reinterpreted as long-period Cepheid in future observations. We consider this possibility in greater detail and study the challenges in making such detections from intervening microlenses.