NASA’s James Webb Space Telescope (JWST) and Chandra X-ray Observatory combined their efforts to look at the Bullet Cluster in a new way, enabling scientists to precisely map the cluster’s dark matter.
Webb’s near-infrared imaging capabilities enabled astronomers to capture the highest detailed images yet of the Bullet Cluster, which comprises a pair of massive galaxy clusters. With Webb’s highly sensitive cameras, researchers can see fainter, more distant galaxies in the Bullet Cluster than ever before.
“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,” says Sangjun Cha, the lead author on a new research paper published this week in The Astrophysical Journal Letters. Cha is a PhD student at Yonsei University in Seoul, South Korea.
“Webb’s images dramatically improve what we can measure in this scene — including pinpointing the position of invisible particles known as dark matter,” adds Kyle Finner, a co-author of the new research paper and an assistant scientist at IPAC at Caltech in Pasadena, California.
As NASA explains, “all galaxies are made up of stars, gas, dust, and dark matter, which are bound together by gravity.” The Bullet Cluster is not just a galaxy, but a grouping of two “very massive collections of galaxies.”
The galaxy clusters, which are massive and therefore have powerful gravitational forces, can act as gravitational lenses that significantly magnify the light of background galaxies. The amount of gravitational lensing, when compared against the amount of visible mass in a cluster, enables scientists to infer the distribution of invisible dark matter.
“Gravitational lensing allows us to infer the distribution of dark matter,” says co-author James Jee, professor at Yonsei University and research associate at UC Davis in California.
It is helpful to think about gravitational lensing and dark matter using a metaphor of a pond filled with crystal-clear water and pebbles, Jee says.
“You cannot see the water unless there is wind, which causes ripples,” the scientist explains. “Those ripples distort the shapes of the pebbles below, causing the water to act like a lens.” This same phenomenon occurs in space, where the water represents dark matter, and the pebbles in the example represent background galaxies.
With Webb’s imaging capabilities, it is much easier to see and measure the galaxies, including the background ones, meaning it is possible to weigh both visible and invisible matter (dark matter) in the galaxy clusters. The researchers also mapped and measured the collective light emitted by intracluster stars. These are stars that are no longer bound to an individual galaxy.
“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 says. If intracluster stars are not bound to galaxies, and instead are bound to dark matter, scientists could learn much more about dark matter and its distribution.
Image credits: NASA, ESA, CSA, STScI, CXC; Science: James Jee (Yonsei University/UC Davis), Sangjun Cha (Yonsei University), Kyle Finner (IPAC at Caltech). Video credits: NASA, ESA, CSA, Joseph DePasquale (STScI)
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