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Astronomie - Dark matter clumps in galaxy clusters bend light surprisingly well

11.09.2020

Not only is the mysterious substance invisible, but it’s also not all where we thought it was

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Observations of 11 galaxy clusters, such as MACSJ1206.2-0847 (shown), reveal that some globs of dark matter in these clusters are denser than expected.

Dark matter just got even more puzzling.

This unidentified stuff, which makes up most of the mass in the cosmos, is invisible but detectable by the way it gravitationally tugs on objects like stars. (SN: 11/25/19). Dark matter’s gravity can also bend light traveling from distant galaxies to Earth — but now some of this mysterious substance appears to be bending light more than it’s supposed to. A surprising number of dark matter clumps in distant clusters of galaxies severely warp background light from other objects, researchers report in the Sept. 11 Science.

This finding suggests that these clumps of dark matter, in which individual galaxies are embedded, are denser than expected. And that could mean one of two things: Either the computer simulations that researchers use to predict galaxy cluster behavior are wrong, or cosmologists’ understanding of dark matter is.

Very high concentrations of dark matter can act like a lens to bend light and drastically alter the appearance of background galaxies as seen from Earth — stretching them into arcs or splitting them into multiple images of the same object on the sky. “It’s totally cool. It’s like a fun house mirror,” says astrophysicist Priyamvada Natarajan of Yale University.

Judging by computer simulations of galaxy clusters, clumps of dark matter around individual galaxies that are dense enough to cause such dramatic gravitational lensingeffects should be rare (SN: 10/4/15). Based on cluster simulations run by Natarajan and colleagues, “we would expect to see 1 [strong lensing] event in every 10 clusters or so,” says study coauthor Massimo Meneghetti, an astrophysicist at the Astrophysics and Space Science Observatory of Bologna in Italy.

But telescope images told a different story. The researchers used observations from the Hubble Space Telescope and the Very Large Telescope in Chile to investigate 11 galaxy clusters from about 2.8 billion to 5.6 billion light-years away. In that set, the team identified 13 cases of severe gravitational lensing by dark matter clumps around individual galaxies. These observations indicate there are more high-density dark matter clumps in real galaxy clusters than in simulated ones, Meneghetti says.

The simulations could be missing some physics that leads dark matter in galaxy clusters to glom tightly together, Natarajan says. “Or … there’s something fundamentally off about our assumptions about the nature of dark matter,” she says, like the notion that gravity is the only attractive force that dark matter feels.

Richard Ellis, a cosmologist at the University College London who was not involved in the work, thinks the crux of the problem is more likely in the computer simulations than in the nature of dark matter. “A cluster of galaxies is a very dangerous place. It’s like the Manhattan of the universe,” he says — busy with galaxies whizzing past one another, colliding and getting torn up. “There’s awful physics that goes into predicting how many of these little lensed things they should find,” Ellis says, so the new result “is intriguing, but my suspicion is that there’s something in the simulations … that isn’t quite right.”

Future observations with the upcoming Euclid space telescope (SN: 11/14/17), the Nancy Grace Roman Space Telescope and Vera C. Rubin Observatory (SN: 1/10/20) could help clear matters up, says Bhuvnesh Jain, an astrophysicist at the University of Pennsylvania who was not involved in the work. “These three telescopes are going to produce extremely large samples of galaxy clusters,” he says. That may lead to a new understanding of the physics in these turbulent environments, and help determine whether unrealistic simulations are to blame for this dark matter mystery. 

Quelle: ScienceNews
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RESEARCHERS FIND A SURPRISING GAP BETWEEN DARK MATTER OBSERVATIONS AND THEORIES

While studying the Coma galaxy cluster in 1933, astronomer Fritz Zwicky uncovered a problem. The mass of all the stars in the cluster added up to only a few percent of the heft needed to keep member galaxies from escaping the cluster's gravitational grip. He predicted that the "missing mass," now known as dark matter, was the glue that was holding the cluster together.

Dark matter, as its name implies, is matter that cannot be seen. It does not emit, absorb, or reflect light, nor does it interact with any known particles. The presence of these elusive particles is only known through their gravitational pull on visible matter in space. This mysterious substance is the invisible scaffolding of our universe forming long filamentary structures—the cosmic web—along which galaxies form.

Even more confounding is that dark matter makes up the vast bulk of the universe's overall mass content. The stuff that stars, planets, and humans are made of accounts for just a few percent of the universe's contents.

Astronomers have been chasing this ghostly substance for decades but still don't have many answers. They have devised ingenious methods to infer dark matter's presence by tracing the signs of its gravitational effects.

One technique involves measuring how dark matter's gravity in a massive galaxy cluster magnifies and warps light from a distant background galaxy. This phenomenon, called gravitational lensing, produces smeared images of remote galaxies and occasionally multiple copies of a single image.

A recent study of 11 hefty galaxy clusters found that some small-scale clumps of dark matter are so concentrated that the lensing effects they produce are 10 times stronger than expected. These concentrations are associated with individual cluster galaxies.

Researchers using the Hubble Space Telescope and the European Southern Observatory's Very Large Telescope in Chile discovered with unprecedented detail smaller-scale distorted images of remote galaxies nested like Matryoshka dolls within the larger-scale lens distortions in each cluster's core, where the most massive galaxies reside.

This unexpected discovery means there is a discrepancy between these observations and theoretical models of how dark matter should be distributed in galaxy clusters. It could signal a gap in astronomers' current understanding of the nature of dark matter.

annotated image of galaxy cluster MACS J1206; galaxies in the three pullouts represent examples of gravitational lensing effects

MACS J1206

Astronomers have discovered that there may be a missing ingredient in our cosmic recipe of how dark matter behaves.

They have uncovered a discrepancy between the theoretical models of how dark matter should be distributed in galaxy clusters, and observations of dark matter's grip on clusters.

Dark matter does not emit, absorb, or reflect light. Its presence is only known through its gravitational pull on visible matter in space. Therefore, dark matter remains as elusive as Alice in Wonderland's Cheshire Cat–where you only see its grin (in the form of gravity) but not the animal itself.

One way astronomers can detect dark matter is by measuring how its gravity distorts space, an effect called gravitational lensing.

Researchers found that small-scale concentrations of dark matter in clusters produce gravitational lensing effects that are 10 times stronger than expected. This evidence is based on unprecedently detailed observations of several massive galaxy clusters by NASA's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope (VLT) in Chile.

Galaxy clusters, the most massive structures in the universe composed of individual member galaxies, are the largest repositories of dark matter. Not only are they held together largely by dark matter's gravity, the individual cluster galaxies are themselves replete with dark matter. Dark matter in clusters is therefore distributed on both large and small scales.

"Galaxy clusters are ideal laboratories to understand if computer simulations of the universe reliably reproduce what we can infer about dark matter and its interplay with luminous matter," said Massimo Meneghetti of the INAF (National Institute for Astrophysics)-Observatory of Astrophysics and Space Science of Bologna in Italy, the study's lead author.

"We have done a lot of careful testing in comparing the simulations and data in this study, and our finding of the mismatch persists," Meneghetti continued. "One possible origin for this discrepancy is that we may be missing some key physics in the simulations."

Priyamvada Natarajan of Yale University in New Haven, Connecticut, one of the senior theorists on the team, added, "There's a feature of the real universe that we are simply not capturing in our current theoretical models. This could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales."

The team's paper will appear in the Sept. 11 issue of the journal Science.

The distribution of dark matter in clusters is mapped via the bending of light, or the gravitational lensing effect, they produce. The gravity of dark matter magnifies and warps light from distant background objects, much like a funhouse mirror, producing distortions and sometimes multiple images of the same distant galaxy. The higher the concentration of dark matter in a cluster, the more dramatic its light bending.

Hubble's crisp images, coupled with spectra from the VLT, helped the team produce an accurate, high-fidelity dark-matter map. They identified dozens of multiply imaged, lensed, background galaxies. By measuring the lensing distortions, astronomers could trace out the amount and distribution of dark matter.

The three key galaxy clusters used in the analysis, MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063, were part of two Hubble surveys: The Frontier Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH) programs.

To the team's surprise, the Hubble images also revealed smaller-scale arcs and distorted images nested within the larger-scale lens distortions in each cluster's core, where the most massive galaxies reside.

The researchers believe that the embedded lenses are produced by the gravity of dense concentrations of dark matter associated with individual cluster galaxies. Dark matter's distribution in the inner regions of individual galaxies is known to enhance the cluster's overall lensing effect.

Follow-up spectroscopic observations added to the study by measuring the velocity of the stars orbiting inside several of the cluster galaxies. "Based on our spectroscopic study, we were able to associate the galaxies with each cluster and estimate their distances," said team member Piero Rosati of the University of Ferrara in Italy.

"The stars' speed gave us an estimate of each individual galaxy's mass, including the amount of dark matter," added team member Pietro Bergamini of the INAF-Observatory of Astrophysics and Space Science in Bologna, Italy.

The team compared the dark-matter maps with samples of simulated galaxy clusters with similar masses, located at roughly the same distances as the observed clusters. The clusters in the computer simulations did not show the same level of dark-matter concentration on the smallest scales–the scales associated with individual cluster galaxies as seen in the universe.

The team looks forward to continuing their stress-testing of the standard dark-matter model to pin down its intriguing nature.

NASA's planned Nancy Grace Roman Space Telescope will detect even more remote galaxies through gravitational lensing by massive galaxy clusters. The observations will enlarge the sample of clusters that astronomers can analyze to further test the dark-matter models.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Quelle: NASA

 

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