Mapping dark matter in the Bullet Cluster using JWST imaging and spectroscopy

¹ Faculty of Mathematics and Physics, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia
² Department of Physics and Astronomy, University of California Davis, 1 Shields Avenue, Davis, CA 95616, USA
³ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV, Groningen, The Netherlands
⁴ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom
⁵ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
⁶ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁷ Department of Physics and Astronomy, York University, 4700 Keele St. Toronto, Ontario M3J 1P3, Canada
⁸ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
⁹ Dunlap Institute for Astronomy and Astrophysics, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹⁰ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111 Playa Ancha, Valparaíso, 2360102, Chile
¹¹ INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
¹² IFPU-Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
¹³ Department of Physics and Astronomy, Ohio University, 1 Ohio University, Athens, OH 45701, USA
¹⁴ Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611, USA
¹⁵ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁶ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
¹⁷ Gemini Observatory, NSF NOIRLab, 670 N. A’ohoku Place, Hilo, Hawai’i, 96720, USA
¹⁸ University of Milan, Department of Physics, Via Celoria 16, I-20133 Milan, Italy
¹⁹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
²⁰ Department of Physics & Astronomy, Tufts University, Medford, MA 02155, USA
²¹ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
²² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
²³ Department of Physics, Department of Astronomy, and CCAPP, The Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210, USA
²⁴ Department of Astronomy and Physics and Institute for Computational Astrophysics, Saint Mary’s University, 923 Robie Street, Halifax, Nova Scotia, B3H 3C3, Canada
²⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone, 00078, Italy

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Received: 29 January 2026
Accepted: 1 April 2026

Abstract

Context. The Bullet Cluster (1E 0657–56), located at a redshift of 0.296, is among the best-known merging galaxy clusters and a key laboratory for dark matter studies. Although the mass distribution in the Bullet Cluster has been modelled using an increasing number of multiply imaged galaxies, only six systems had spectroscopic redshifts published prior to this work, which are essential for system confirmation and as lens model constraints.

Aims. We present an updated gravitational lens model of the Bullet cluster, obtained by combining JWST NIRCam imaging and NIRSpec spectroscopy. Our lens model has been constrained on the basis of a catalogue of 135 secure multiple images from 27 background galaxies with spectroscopic redshifts, uniformly covering both subclusters and a wide redshift range of 0.9–6.7. We also provide a catalogue of 199 multiple image candidates.

Methods. We modelled the cluster with the parametric lens modelling code Lenstool. We incorporated several large-scale halos, cluster member galaxies, intracluster gas, and group-scale halos surrounding the cluster core, motivated by spectroscopic studies of cluster member kinematics.

Results. We describe the main cluster component with a complex, elongated double-peaked distribution, along with the subcluster, modelled using a single large-scale halo coinciding closely with the brightest cluster galaxy at a projected separation of 4⁺³₋₂ kpc. The uncertainty of the displacement has been improved three-fold thanks to the addition of JWST systems. The addition of group-scale substructures, roughly following the two axes of cluster assembly, improves the fit to the multiple image positions and provides a physically motivated alternative to a constant shear component. Our lens model shows the closest agreement with previous studies in aperture mass profiles at ∼60 kpc from the brightest cluster galaxies (BCGs), but exhibits significant differences in the detailed mass distribution as a result of different lens-modelling strategies and adopted constraints. The differences are reflected in small, but spatially coherent deviations between the new spectroscopic redshifts and redshifts predicted by earlier lens models.