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The Perseus galaxy cluster taken by the Subaru Telescope on the Mauna Kea Observatory in Hawaii. The newly found subcluster is on the suitable. Overlaid in blue is the inferred distribution of darkish matter. . | Credit score: HyeongHan et al.
Astronomers thought the Perseus cluster was a massive-but-stable grouping of galaxies, till they discovered hints of a collision with one other cluster — however nobody had recognized a cosmic interloper. Now, utilizing a way referred to as weak gravitational lensing, scientists suppose they’ve lastly discovered the hidden intruder.
The Perseus galaxy cluster is without doubt one of the most large constructions within the recognized universe. Bearing the title of its host constellation, it is a huge grouping of hundreds of galaxies unfold throughout 11.6 million gentle years, roughly 100 occasions the diameter of the Milky Approach.
At 250 million light-years from Earth, it is comparatively close by, although it is shifting away at 3,335 miles per second (5,366 kilometers per second) as a result of expansion of the universe.
A ‘relaxed’ galaxy?
Astronomers name calm and secure galaxy clusters “relaxed,” which implies they have not collided with different clusters within the latest historical past of the universe. For years, the Perseus cluster served as a chief instance of a relaxed cluster.
Regular flows of scorching fuel transfer between its element galaxies and lose warmth because the fuel sinks towards the cluster’s heart. There’s additionally a faint glow of radio waves, referred to as a radio “mini-halo,” surrounding the central galaxy. Each are indicators of a secure cosmic surroundings.
But astronomers observed that as a substitute of being spherical like an undisturbed cluster must be, it was lopsided in an east-west course, hinting that one thing was amiss.
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Then, in 2012, astronomers noticed “chilly fronts” within the cluster — large, light-years-long areas that seemingly kind when galaxy clusters collide. These fronts mark the boundary the place scorching fuel from one cluster slams into cooler, denser fuel from one other. It was a powerful clue that Perseus had been in a serious cosmic crash. But when that was true, a query remained: The place was the opposite cluster that prompted it?
‘Smoking gun’ proof
Within the new analysis, astronomers based mostly within the U.S. and South Korea suppose they’ve discovered the “smoking gun” within the case of the clandestine cosmic collision. In a brand new paper printed within the journal Nature Astronomy, the researchers clarify how they used a way referred to as weak gravitational lensing to identify the stays of a cluster that collided with the Perseus galaxy cluster way back.
Albert Einstein predicted the phenomenon of gravitational lensing over 100 years in the past, whereby large objects like galaxies would warp space-time, bending gentle rays and magnifying distant sources. The quantity that an object deflects gentle offers astronomers a deal with on its mass, and scientists have since found many incredible examples of gravitational lensing.
This will produce magnified photographs of distant objects and ‘Einstein rings’. | Credit score: Credit score: P. van Dokkum et al., Nature Astronomy accepted, 2023
In weak gravitational lensing, the picture of background galaxies is barely barely distorted because it passes by the cluster on its technique to us. Utilizing the Subaru Telescope in Hawaii, the astronomers within the new research analyzed the distortion of sunshine from galaxies far behind the Perseus cluster.
“As we have no idea the intrinsic form of every galaxy, we can not infer how a lot the picture of the background galaxies are distorted,” HyeongHan Kim, a graduate pupil at Yonsei College in South Korea and first creator of the paper, advised Reside Science in an electronic mail.
But, by viewing a lot of background galaxies, the staff labored out the typical distortion and used a pc simulation to determine the quantity of mass within the foreground cluster. Additionally they pinpointed the place the mass needed to be with a purpose to produce the distortions they noticed.
The astronomers found an enormous area of each seen and darkish matter, referred to as a subcluster halo, surrounding a smaller grouping of galaxies. This area was centered on galaxy NGC 1264 and embedded within the outskirts of the Perseus cluster. About 100 occasions the mass of the Milky Approach, the subcluster is linked to the primary galaxy cluster by a “mass bridge” that’s about 1.4 million light-years lengthy and has virtually the identical mass.
A determine from the paper displaying the distribution of darkish matter (white contours) and galaxies (inexperienced contours) within the Perseus cluster. The purple line signifies the situation of a giant chilly entrance. | Credit score: HyeongHan et al.
The pc mannequin steered that this bridge is direct proof of a gravitational interplay between the 2 clusters, fairly than an opportunity alignment, and that it coincides with the lopsided form of the cluster. The staff’s pc mannequin additionally re-created the chilly fronts found in 2012.
“The outcome shocked me, as a result of I thought-about Perseus to be a relaxed cluster,” Kim stated.
Their simulation begins round 7.5 billion years in the past, when the merging subcluster fell throughout the affect of the Perseus cluster. It took about 2 billion years for this subcluster to cross close to the middle of the primary cluster for the primary time. Then, slowed down and pulled again by gravity, it handed by once more 3 billion years later. This cosmic dance repeated, and almost 2 billion years later, the subcluster handed by a 3rd time, roughly 750 million years in the past.
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The findings are vital as a result of the area is a chief goal for astronomers who’re finding out how galaxy clusters kind and evolve.
“The Perseus cluster, because of its proximity, has been extensively investigated,” Kim stated, “permitting us to find and check many vital astrophysical processes.” So though this research focuses on a single cluster, it has large implications for our understanding of the universe, he added.
As well as, the outcomes spotlight the ability of weak gravitational lensing, particularly the place conventional strategies have didn’t reveal the unseen universe. This method not solely helps to disclose hidden constructions within the universe but in addition opens the door to discovering extra galaxy-cluster mergers and to understanding how these large methods take form.
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