The James Webb Space Telescope has delivered the clearest, deepest images yet of the Bullet Cluster, unveiling thousands of faint, distant galaxies and offering the most precise map of dark matter in this iconic colliding galaxy cluster.
“With Webb’s observations, we carefully measured the mass of the Bullet Cluster with the largest lensing dataset to date, from the galaxy clusters’ cores all the way out to their outskirts,” said lead author Sangjun Cha, a PhD student at Yonsei University in Seoul.
Previous studies relied on less lensing data, leading to less precise estimates of the system’s mass.
“Webb’s images dramatically improve what we can measure in this scene – including pinpointing the position of invisible particles known as dark matter,” said co-author Kyle Finner, an assistant scientist at Caltech.
How light shows the dark
The Bullet Cluster is made of two massive galaxy clusters bound by gravity. It acts as a natural gravitational lens that magnifies background galaxies.
James Jee, a professor at Yonsei University and research associate at UC Davis, is a co-author of the study. “Gravitational lensing allows us to infer the distribution of dark matter,” he said.
To visualize this effect, Jee compares it to ripples on a pond. You can’t see the clear water unless there are ripples that distort the shapes of the pebbles below – just as dark matter distorts the light from galaxies behind it.
By measuring thousands of galaxies, the scientists used Webb’s images to weigh visible and invisible mass in the cluster.
The team also mapped the faint glow of intracluster stars – those not bound to any single galaxy. These drifting stars may closely trace dark matter.
Webb’s dark matter reveal
Webb’s observations produced a layered view, combining near-infrared data with X-ray imagery from NASA’s Chandra X-ray Observatory, revealing hot gas in pink, the bullet shape in the cluster, and the newly refined dark matter distribution in blue.
“We confirmed the intracluster light can be a reliable tracer of dark matter, even in a highly dynamic environment like the Bullet Cluster,” Cha said.
This strengthens the case that these unbound stars closely trace dark matter’s invisible scaffolding.
The new map reveals detailed structure, including an asymmetric mass region on the left side of the larger cluster. This feature indicates prior collisions that have left behind signatures in the distribution of matter.
Mysterious nature of dark matter
Dark matter does not emit, reflect, or absorb light, making it notoriously difficult to study. Yet the Bullet Cluster offers a rare laboratory, showing dark matter separated from hot gas during a cosmic collision while still aligning with the galaxies.
“As the galaxy clusters collided, their gas was dragged out and left behind, which the X-rays confirm,” Finner explained. “Webb’s observations show that dark matter still lines up with the galaxies – and was not dragged away.”
The results set tighter constraints on the possibility of dark matter particles interacting with each other. They support theories that dark matter passes through itself without friction, consistent with its mysterious and ghostly nature.
Clues to a chaotic past
The elongated mass and clumps in the new map hint at a more complex history for the Bullet Cluster, suggesting multiple collisions over billions of years.
“A more complicated scenario would lead to a huge asymmetric elongation like we see on the left,” Jee said.
The Bullet Cluster, located in the Carina constellation 3.8 billion light-years from Earth, is so massive that even Webb’s powerful NIRCam could only capture part of it.
“It’s like looking at the head of a giant,” Jee explained. “Webb’s initial images allow us to extrapolate how heavy the whole ‘giant’ is, but we’ll need future observations of the giant’s whole ‘body’ for precise measurements.”
Future missions, deeper maps
NASA’s upcoming Nancy Grace Roman Space Telescope, launching by May 2027, will complement Webb’s discoveries with wide-field near-infrared imaging.
The Roman telescope will enable complete mass estimates of the Bullet Cluster and allow scientists to simulate its ancient collision in detail.
“With Roman, we will have complete mass estimates of the entire Bullet Cluster, which would allow us to recreate the actual collision on computers,” Finner said.
Through these detailed observations, Webb is enriching our understanding of dark matter. It’s also advancing knowledge of how massive structures form and evolve, offering a clearer view of the hidden forces shaping the cosmos.
The study is published in the Astrophysical Journal Letters.
Image Credit: NASA, ESA, CSA, STScI, CXC
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