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

TL;DR

This study delivers an updated strong-lensing model of the Bullet Cluster by integrating JWST NIRCam imaging and NIRSpec spectroscopy, increasing the catalog of spectroscopically confirmed lensed systems to 135 images from 27 galaxies (redshifts $0.9<z<6.7$) and providing 199 image candidates. Using Lenstool with a multi-component mass model (large-scale PIEMD halos, cluster members with scaling relations, fixed intracluster gas, and group-scale substructures), the authors map a complex, double-peaked main cluster and a single-halo subcluster, achieving a threefold improvement in the halo–galaxy alignment precision thanks to the JWST data. They demonstrate that including physically motivated group-scale substructures yields a better fit to the inner multiple images than a constant external shear, while preserving consistency with aperture-mass profiles near the BCGs. The work also contrasts the new JWST-based model with prior lens models, highlighting spatially coherent redshift deviations and underscoring the critical impact of spectroscopic constraints for accurate mass reconstructions and implications for dark matter studies, including self-interaction constraints.

Abstract

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.

Figures (16)

• Figure 1: JWST/NIRCam image of the Bullet cluster with multiply imaged systems. The RGB image is composed of several NIRCam filters (F444W, F277W and F356W in red, F200W and F150W in green, and F115W and F090W in blue). A smoothed Chandra X-ray image, showing cluster gas, is shown in pink. The colour of multiple images represents their quality grades (gold, silver or bronze) and whether they were used in the pre-JWST models (Richard21, Paraficz16) or were included after JWST observations (this work or cha25). Squares represent systems which have known pre-JWST redshifts from the literature, diamonds represent images for which we obtained new spectroscopic redshifts. Circles indicate multiple images for which we did not measure spectroscopic redshifts directly, but have new $z_{\rm sys}$ from other images of the system. Stars represent newly identified multiple-image candidates without spectroscopic redshifts. For clarity, we show only one multiply lensed feature per galaxy. The red solid line represents the longest tangential critical curves from our lensing model for redshift $z=9$.
• Figure 2: Spectroscopic redshift distribution of multiply lensed galaxies in the gold catalogue. The grey colour indicates the spectroscopic redshifts used in previous studies (cha25).
• Figure 3: The three multiple images of system K17. Left: NIRCam cutout centered on the cluster member G5. Middle: BCG-subtracted cutout revealing multiple images K17.2 and K17.3. Right: Cutout showing multiple image K17.1.
• Figure 4: The inverted grayscale image (F277W) of the Bullet cluster covering the full NIRCam FOV, with indicated photometric and spectroscopic cluster members and other halos included in the lens model. The cluster members with existing $z_{\rm spec}$ data from the literature are shown in blue, those with new NIRSpec redshifts in magenta, and those selected photometrically in violet. The positions of other lens model components (halos) are indicated with empty circles if they have a fixed position, and "X" if their position was left as a free parameter (in which case we mark the best-fit position). Galaxy-scale halos, modelled outside the cluster member scaling relations, and large-scale halos are shown in orange and red, respectively. For halos that are outside the NIRCam FOV, we indicate the direction of their positions with arrows.
• Figure 5: Left panel: Convergence (total density) from our best-fit fiducial model, covering a large FOV with indicated positions of substructure halos with fixed (red) or free (orange) normalisation $\sigma_{\rm lt}\,$. The contour values indicate the values of $\kappa$ while the colourmap shows $\log \kappa$. Lower right panel: Convergence map of the inner regions with multiple images. The colour of multiple images represents their contribution to the goodness of fit $\chi^2$. Upper right panel: Convergence map of the main cluster (left) and the subcluster (right) with indicated BCG positions.