NASA’s James Webb Space Telescope recently zeroed in on the Bullet Cluster — delivering highly detailed images that show a greater abundance of extremely faint and distant galaxies than ever before. Using Webb’s crisp near-infrared observations of this region, researchers have more completely mapped the colliding galaxy clusters’ contents.
“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 Sangjun Cha, the lead author of the paper published in The Astrophysical Journal Letters and a PhD student at Yonsei University in Seoul, South Korea. (Previous studies of the Bullet Cluster with other telescopes relied on significantly less lensing data, which netted out with 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 Kyle Finner, a co-author and an assistant scientist at IPAC at Caltech in Pasadena, California.
Image A: Bullet Cluster (Webb and Chandra Image)
Mapping the Dark Matter
All galaxies are made up of stars, gas, dust, and dark matter, which are bound together by gravity. The Bullet Cluster is made up of two very massive collections of galaxies, known as galaxy clusters, that are themselves bound by gravity.
These galaxy clusters act as gravitational lenses, magnifying the light of background galaxies. “Gravitational lensing allows us to infer the distribution of dark matter,” said James Jee, a co-author, professor at Yonsei University, and research associate at UC Davis in California.
To visualize gravitational lensing and dark matter, think of a pond filled with clear water and pebbles. “You cannot see the water unless there is wind, which causes ripples,” Jee explained. “Those ripples distort the shapes of the pebbles below, causing the water to act like a lens.” The same thing happens in space, but the water is dark matter and the pebbles are background galaxies.
In all, the team measured thousands of galaxies in Webb’s images to accurately “weigh” both the visible and invisible mass in these galaxy clusters. They also carefully mapped and measured the collective light emitted by stars that are no longer bound to individual galaxies — known as intracluster stars.
The revised map of the Bullet Cluster is shown in a new image: Layered on top of an image from Webb’s NIRCam (Near-Infrared Camera) is data from NASA’s Chandra X-ray Observatory that shows hot gas in pink, including the bullet shape at right. Refined measurements of the dark matter, calculated by the team using Webb’s observations, are represented in blue. (See the defined galaxy clusters within the dashed circles.)
Their findings are persuasive: “We confirmed that the intracluster light can be a reliable tracer of dark matter, even in a highly dynamic environment like the Bullet Cluster,” Cha said. If these stars are not bound to galaxies, but to the cluster’s dark matter, it might become easier to pin down more specifics about the invisible matter.
Viewed as a whole, the researchers’ new measurements significantly refine what we know about how mass is spread throughout the Bullet Cluster. The galaxy cluster on the left has an asymmetric, elongated area of mass along the left edge of the blue region, which is a clue pointing to previous mergers in that cluster.
Dark matter does not emit, reflect, or absorb light, and the team’s findings indicate that dark matter shows no signs of significant self-interaction. If dark matter did self-interact in Webb’s observations, the team would see an offset between the galaxies and their respective dark matter.
“As the galaxy clusters collided, their gas was dragged out and left behind, which the X-rays confirm,” Finner said. Webb’s observations show that dark matter still lines up with the galaxies — and was not dragged away.
Although earlier measurements with other telescopes also identified invisible mass in addition to the mass in the galaxies, it was still possible that the dark matter could interact with itself to some degree. These new observations place stronger limits on the behavior of dark matter particles.
Image B: Bullet Cluster (NIRCam Image)
‘Replaying’ the Collision
The strange new clumps and elongated line of mass the team identified may mean that the Bullet Cluster was produced by more than one collision of galaxy clusters billions of years ago.
The larger cluster, which now sits on the left, might have suffered a minor collision before it rammed through the galaxy cluster now at right. The same larger cluster may also have experienced a violent interaction afterward, causing an additional shake up of its contents. “A more complicated scenario would lead to a huge asymmetric elongation like we see on the left,” Jee said.
Video A: Bullet Cluster Hubble to Webb Fade
Video: NASA, ESA, CSA, Joseph DePasquale (STScI)
The Head of a ‘Giant’
The Bullet Cluster is huge, even in the vast expanse of space. Webb’s NIRCam covered a significant portion of the hulking debris with its images, but not all of it. “It’s like looking at the head of a giant,” said Jee. “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.”
In the near future, researchers will also have expansive near-infrared images from NASA’s Nancy Grace Roman Space Telescope, which is set to launch by May 2027. “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.
The Bullet Cluster is found in the Carina constellation 3.8 billion light-years from Earth.
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
To learn more about Webb, visit:
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Media Contacts
Laura Betz - laura.e.betz@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Claire Blome - cblome@stsci.edu
Space Telescope Science Institute, Baltimore, Md.
Christine Pulliam - cpulliam@stsci.edu
Space Telescope Science Institute, Baltimore, Md.
Related Information
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