A very recent study uses the sharp images of JWST to provide the most detailed mass reconstruction of the *Bullet Cluster (1E 0657-56) to date, an extraordinary system consisting of two colliding galaxy clusters.
Imagine a colossal cosmic collision, powerful enough to separate dark matter (DM) from the intracluster medium (ICM, hot gas visible in X-rays) and galaxies. The Bullet Cluster is a cosmic laboratory that helps us understand one of the greatest mysteries of the universe: dark matter.
Scientists have reconstructed the mass distribution of the Bullet Cluster with unprecedented precision, using a technique called gravitational lensing (strong and weak lensing). This method exploits the distortion of light from distant galaxies to map DM, which neither emits nor absorbs light.
With 146 multiple images and a very high data density, the team created a detailed map without assuming that the light of the galaxies traces the mass, that is, the team avoided making preconceived assumptions about the relationship between visible light and total mass.
This approach allowed the team to discover a complex structure: the main cluster of the Bullet Cluster is elongated (northwest-southeast) and has at least three subclumps of mass, aligned with the brightest cluster galaxies (Brightest Cluster Galaxies, BCGs).
The subcluster (or secondary cluster of smaller mass), on the other hand, is more compact, with a single peak of mass, but shows a “tail” towards the east, which could be a bridge of dark matter towards the main cluster.
The intracluster light (ICL), the light scattered between the galaxies, follows almost perfectly the distribution of dark matter, with a distance of only ~20 kpc (a very low value!). This confirms that the ICL is an excellent “tracer” of dark matter, even in a chaotic system like this.
The separation between dark matter and gas (visible in X-rays) is clear: ~150 kpc (~489,234 ly) in the subcluster and up to 400 kpc (~1,304,624 ly) in the main cluster. These offsets, together with a radio halo that follows the mass rather than the gas, suggest that the Bullet Cluster is not the result of a simple merger between two clusters, but of a more complex process.
The study provides one of the most stringent limits on the possibility that DM interacts with itself, but more data are needed to confirm the estimates. This means that, thanks to the precise observations of JWST, the researchers have established that any interactions between DM particles are extremely weak, placing a very strict limit on how much these interactions can occur. This is an important step in understanding what DM is made of, but more observations are still needed!
The Bullet Cluster is a key piece of evidence for the existence of DM and a test case for the standard cosmological model (ΛCDM). This study shows that its merger history is more intricate than scientists thought, forcing them to rethink numerical simulations. It also demonstrates the power of JWST to reveal hidden cosmic structures, paving the way for new discoveries about the nature of dark matter.
What's next?
The limited field of view of JWST does not allow scientists to calculate the total mass of the system, but future studies with wide-field data (e.g. DECam) will complete the picture. The Bullet Cluster continues to be a unique laboratory for exploring the invisible Universe!
The Bullet Cluster is located in the Carina constellation, 3.8 billion light-years away.
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Image description: Central region of the Bullet Cluster, captured by JWST in infrared light, with galaxies and stars scattered across a black background. The hot gas (pink, from Chandra X-ray) shows the "bullet" on the right, while the dark matter (blue) is separated, highlighting the unique dynamics of the collision between two galaxy clusters.
Image credit: NASA, ESA, CSA, STScI, CXC; Science: James Jee (Yonsei University/UC Davis), Sangjun Cha (Yonsei University), Kyle Finner (IPAC at Caltech)
Reference ➡️ NASA Webb ‘Pierces’ Bullet Cluster, Refines Its Mass
Paper➡️ A High-Caliber View of the Bullet Cluster through JWST Strong and Weak Lensing Analyses
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* The Bullet Cluster consists of two colliding galaxy clusters: the main cluster, which is more massive, and the subcluster (or the secondary cluster of smaller mass), famous for its bullet-shaped shock front observed in X-rays. The subcluster shows a 'tail' of mass extending toward the main cluster, suggesting a complex merger story.
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