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

We present an updated gravitational lens model of the Bullet cluster (1E 0657-56) by combining JWST NIRCam imaging and NIRSpec spectroscopy. Although previous lens models relied on many multiply imaged galaxies, only six systems had spectroscopic redshifts prior to this work. Our lens model is constrained by 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. We model the cluster with Lenstool and incorporate several large-scale haloes, cluster members, the intracluster gas, and group-scale haloes surrounding the cluster core, motivated by spectroscopic studies of cluster member kinematics. We describe the main cluster component with a complex, elongated double-peaked distribution, and the subcluster with a single large-scale halo aligning closely with the brightest cluster galaxy ($4_{-2}^{+4}$ kpc). The uncertainty of the alignment is improved threefold with 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 constant shear. Our lens model shows the closest agreement with previous studies in aperture mass profiles at $\sim60$ kpc from the 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.

💡 Research Summary

The Bullet Cluster (1E 0657‑56) is a textbook example of a high‑velocity, post‑collision merger of two galaxy sub‑clusters, making it a cornerstone for studies of dark matter (DM) distribution and self‑interaction. Prior strong‑lensing models relied on Hubble Space Telescope imaging and a handful of spectroscopic redshifts (six systems), limiting the precision of mass reconstructions and consequently the robustness of DM constraints. In this work the authors combine deep JWST NIRCam imaging (eight filters from 0.9 to 5 µm, ∼6.4 ks per filter) with NIRSpec PRISM spectroscopy (R≈100, 0.6–5.3 µm) obtained in GO 4598 (2025). The imaging was processed with the CANUCS pipeline, drizzled to 40 mas pixels, and the intracluster light and bright cluster galaxies were modelled and subtracted to enhance faint arc detection.

From the NIRCam data the team identified a large number of multiple‑image candidates, guided by predictions from the previous Richard et al. 2021 (R21) model. Spectroscopic follow‑up with NIRSpec yielded secure redshifts for 135 multiple images belonging to 27 distinct background galaxies, covering a redshift range of 0.9–6.7. The typical redshift uncertainty is ∆z/(1+z)≈0.002, an order of magnitude better than earlier measurements. The final catalogue includes quality grades (gold, silver, bronze) and a supplementary list of 199 candidate images.

The lens model is built with the parametric code Lenstool. It incorporates: (1) two large‑scale halos describing a double‑peaked, elongated mass distribution for the main cluster; (2) a single large‑scale halo for the sub‑cluster that aligns with its brightest cluster galaxy (BCG) within 4 kpc (−2 + 4 kpc); (3) galaxy‑scale halos for individual cluster members, scaled by their F277W luminosities; (4) a gas halo traced by X‑ray emission; and (5) ten group‑scale halos placed along the two axes of cluster assembly identified in previous kinematic studies (Benavides et al. 2023). The inclusion of these group‑scale substructures, rather than a constant external shear term, significantly reduces the positional residuals of the multiple images, indicating that the model captures physically motivated mass components.

Compared with earlier models, the new reconstruction reproduces aperture mass profiles at ∼60 kpc from each BCG, but shows notable differences in the inner mass distribution. These differences manifest as small, spatially coherent offsets between the spectroscopic redshifts measured here and the redshifts predicted by older models, underscoring the impact of the expanded spectroscopic sample and the additional sub‑structures. The alignment uncertainty of the sub‑cluster’s mass peak is improved by a factor of three thanks to the JWST constraints.

The refined mass map has immediate implications for DM self‑interaction studies. Earlier limits (σ/m < 0.2 cm² g⁻¹) derived from the Bullet Cluster relied on the separation between the DM peak and the X‑ray gas. The present model provides a more precise measurement of that separation and captures asymmetries at larger radii, which are essential for robust σ/m constraints. Moreover, the catalog of high‑redshift, highly magnified sources (including a z≈11 arc) and the accurate magnification estimates will enable detailed studies of early galaxy formation and the interstellar medium at z∼2 with ALMA.

In summary, the authors deliver the most spectroscopically constrained strong‑lensing model of the Bullet Cluster to date, leveraging JWST’s unprecedented imaging depth and spectroscopic capability. The model’s inclusion of multiple large‑scale, galaxy‑scale, gas, and group‑scale components yields a nuanced picture of the cluster’s dark matter distribution, reduces systematic uncertainties, and sets the stage for tighter constraints on dark matter physics and for a broad range of high‑redshift astrophysical investigations.