Title: A High-Caliber View of the Bullet Cluster Through JWST Strong and Weak Lensing Analyses
Authors: Sangjun Cha, Boseong Young Cho, Hyungjin Joo, Wonki Lee, Kim HyeongHan, Zachary P. Scofield, Kyle Finner, M. James Jee
First Author’s Institution: Department of Astronomy, Yonsei University, Seoul, Korea
Status: Submitted to The Astrophysical Journal, available on arXiv
Despite the extreme depths and resolutions of the James Webb Space Telescope (JWST), there’s still no smoking gun to be found in the hunt for dark matter. Yet, the extremely high-velocity collision in today’s paper might aid in the search for what this mysterious material is made of. The target of today’s paper is “the Bullet Cluster”, and despite its name, it is not one, but actually two massive clusters of galaxies gravitationally interacting with one another. Due to the high speed with which the smaller “bullet” subcluster collided and passed through the larger one, a strong bow shock caused the gas in the system to compress and heat to millions of degrees. Typically, this alone would make this system interesting to astronomers, but the real kicker here is that this collision was powerful enough to separate the dark matter from the normal, luminous matter. This has made the Bullet Cluster an important testing ground for theories on the nature of dark matter and is exactly what the authors of today’s paper attempt to use it for.
Mapping Hidden Mass
The authors used JWST imaging, which captured the majority of the Bullet Cluster system, to obtain high-resolution, multi-wavelength data. They then identified over 146 areas in the image subject to strong lensing, which is when the light from a background galaxy is bent around mass in the foreground Bullet Cluster system, since that mass distorts spacetime. They also obtained a weak lensing model for the image through statistical analysis, since the effects of weak lensing, while similar, are much more subtle than strong lensing. They then created a mass map for the system by feeding their strong lensing + weak lensing model into a reconstruction algorithm. Essentially, this algorithm creates the most likely distribution of mass, both dark matter and normal matter, that would re-create all the lensing that they observed.
What’s special about the algorithm that they used here is that it does not assume that light traces mass. In general, this assumption is correct, so other algorithms use it to get the most accurate mass estimate. But the authors here wanted to confirm that even in extreme environments like the Bullet Cluster that this is still the case, so they sent their algorithm in “blind”.
Comparing How the Light Traces the Dark
Once they had their mass map, which is dominated mostly by the dark matter in the system, they could compare it to the optical light between the system’s galaxies, referred to as the intra-cluster light (ICL). They used a statistical test called the modified Hausdorff distance to determine how similar the distribution of ICL and dark matter were to each other. They found that, despite the extreme gravitational interactions going on in the Bullet Cluster, the ICL still does an excellent job overall of tracing the system’s mass (see Figure 2).
Next, they compared their mass map to the distributions of hot plasma, as observed in X-ray wavelengths with Chandra, and of Hydrogen gas, as observed in radio wavelengths with MeerKAT (see Figure 1). They found that the dark matter was offset from both of these distributions, which was already known about this system. However, their measurement of these mass offsets indicated that it is unlikely that the Bullet Cluster was formed from a merger between just two galaxy clusters. The authors suggest then that the Bullet Cluster’s formation is much more complicated than previously thought but aren’t able to give any guesses about the initial conditions.
What a Massive Cluster Tells Us About a Tiny Particle
Finally, the authors were able to use their mass map to put some constraints on one of the theories of dark matter: self-interacting dark matter (SIDM). If dark matter particles interact with each other, then they have a cross-section which can be calculated. Here, the authors calculate it from the offset in the position of the most massive galaxy in the Bullet Cluster and the position of the highest density of dark matter, about 18 kpc (see Figure 2). They were able to determine that the upper limit of this cross section is 0.5 cm^2/g. This is the tightest constraint on SIDM’s cross section to date, but we still have that pesky “per gram” in our units since we don’t know the mass of a dark matter particle (but this is being studied!).
This cross section is on the lower limits of what is thought to be a reasonable range for self-interacting dark matter theories (~ 0.5-2 cm^2/g), but certainly doesn’t rule anything out. For further context, when dividing by mass, the cross section of a hydrogen atom is ~ 1.7×10^7 cm^2/g, which is much bigger, but we would expect that when comparing an atom to a potential subatomic dark matter particle.
Altogether, the authors of this paper were able to use both the precision of JWST and the impartiality of their mass reconstruction algorithm to learn new things about both the structure of the Bullet Cluster and the nature of dark matter. Although there are still some open questions that will need to be answered, this study certainly is not just a shot in the dark!
Astrobite edited by Maggie Verrico
Featured image credit: Background from Figure 5 in Cha et al. (2025), water gun from Yasin Arıbuğa
