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Something’s Just Not Right with our Current Understanding of Dark Matter
This is an image of Galaxy Cluster Abell 370, located about 4 billion light-years away, which contains an astounding assortment of several hundred galaxies tied together by the mutual pull of gravity. Entangled among the galaxies are mysterious-looking arcs of blue light. These are actually distorted images of remote galaxies behind the cluster. These far-flung galaxies are too faint for Hubble to see directly. Instead, the gravity from the cluster acts as a huge lens in space that magnifies and stretches images of background galaxies like a funhouse mirror. Nearly 100 distant galaxies have multiple images caused by the lensing effect. The most stunning example is "the Dragon," an extended feature that is probably several duplicated images of a single background spiral galaxy stretched along an arc. Astronomers chose Abell 370 as a target for Hubble because its gravitational lensing effects can be used for probing remote galaxies that inhabited the early Universe. (Image Credit: NASA, ESA, and J. Lotz and the HFF Team - STScI)
Something’s Just Not Right with our Current Understanding of Dark Matter
Dark matter does not emit, absorb, or reflect light. Its presence, if it truly exists, is known only through its gravitational pull on visible matter in space. This mysterious substance is believed to be the invisible scaffolding of our Universe, forming long filamentary structures -- the cosmic web -- along which galaxies form. 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 those clusters. One way astronomers can detect dark matter is by measuring how its gravity distorts space through an effect called “gravitational lensing.” Researchers have found that small-scale concentrations of dark matter in clusters produce gravitational lensing effects that are ten times stronger than expected. This evidence is based on unprecedented 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.
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 referred to as dark matter, was the glue that was holding the cluster together.
Even more confounding is that dark matter is believed to make 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 eleven 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 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.
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 believed to be held together largely by dark matter's gravity, but the individual cluster galaxies are themselves believed to be 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, 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 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.
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.
Gravitational Lensing – Like Looking Through a Giant Magnifying Glass
When taken to the extreme, gravity can create some intriguing visual effects that Hubble’s is well suited to observing. Einstein’s general theory of relativity describes how mass concentrations distort the space around them. A gravitational lens can occur when a huge amount of matter, like a cluster of galaxies, creates a gravitational field that distorts and magnifies the light from distant galaxies that are behind it but in the same line of sight. The effect is like looking through a giant magnifying glass. It allows researchers to study the details of early galaxies too far away to be seen with current technology and telescopes.
Smaller objects, like individual stars, can also act as gravitational lenses when they pass in front of more distant stars. For a few days or weeks, light from the more distant star temporarily appears brighter because it is magnified by the gravity of the closer object. This effect is known as gravitational micro-lensing.
The simplest type of gravitational lensing occurs when there is a single concentration of matter at the center, such as the dense core of a galaxy. The light of a distant galaxy is redirected around this core, often producing multiple images of the background galaxy. When the lensing approaches perfect symmetry, a complete or almost complete circle of light is produced, called an Einstein ring. Hubble observations have helped to greatly increase the number of Einstein rings known to astronomers.
More complex gravitational lensing arises in observations of massive clusters of galaxies. While the distribution of matter in a galaxy cluster generally does have a center, it is never circularly symmetric and can be significantly lumpy. Background galaxies are lensed by the cluster and their images often appear as short, thin “lensed arcs” around the outskirts of the cluster.
These lensed images also act as probes of the matter distribution in the galaxy cluster. The results indicate that most of the matter in a galaxy cluster is not in the visible galaxies or hot gas around them and does not emit light, and is thus called dark matter. The distribution of lensed images reflects the distribution of all matter, both visible and dark. Hubble’s images of gravitational lensing have been used to create maps of dark matter in galaxy clusters.
In turn, a map of the matter in a galaxy cluster helps provide better understanding and analysis of the gravitationally lensed images. A model of the matter distribution can help identify multiple images of the same galaxy or predict where the most distant galaxies are likely to appear in a galaxy cluster image. Astronomers work between the gravitational lenses and the cluster matter distribution to improve our understanding of both.
Because very distant galaxies are very faint, gravitational lenses extend Hubble’s view deeper into the Universe. Gravitational lensing not only distorts the image of a background galaxy, it can amplify its light. Looking through a lensing galaxy cluster, Hubble can see fainter and more distant galaxies than otherwise possible. It is like having an extra lens that is the size of the galaxy cluster. The Frontier Fields project has examined multiple galaxy clusters, measured their lensing and matter distribution and identified a collection of these most distant galaxies.
The diverse, lensed images of crosses, rings, arcs and more are both intriguing and informative. Gravitational lensing probes the distribution of matter in galaxies and clusters of galaxies, and enables observations of the distant Universe. Hubble’s data will also provide a basis and guide for the James Webb Space Telescope, whose infrared observations will push yet farther into the cosmos.
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.
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