Observations by NASA’s European Space Agency’s Hubble Space Telescope and the European Southern Observatory’s very large telescope (VLT) in Chile have found that something may be missing from theories about how dark matter behaves.
This missing ingredient may explain why the researchers found an unexpected discrepancy between observations of dark matter concentrations in a sample of massive galactic clusters and theoretical computer simulations of how dark matter should be distributed in clusters. New findings show that some small concentrations of dark matter produce lensing effects that are 10 times stronger than expected.
Dark matter is the invisible glue that holds stars, dust and gas together in a galaxy. This mysterious substance makes up most of the mass of the galaxy and forms the basis of the large-scale structure of our universe. Because dark matter does not emit, absorb, or reflect light, its presence is known only by the gravitational attraction of visible matter into space. Astronomers and physicists are still trying to figure out what that is.
Galaxy clusters are the largest repositories of dark matter. The clusters are made up of individual member galaxies that are held together largely by the gravity of dark matter.
“There is a feature of the real universe that we simply do not capture in our current theoretical models,”; said Priyamwada Natarajan of Yale University in Connecticut, USA, one of the team’s greatest theorists. “This could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have allowed us to explore the detailed distribution of dark matter on the smallest scale.”
How is dark matter mapped?
The distribution of dark matter in clusters is mapped by measuring the bending of light – the effect of the gravitational lens – that they produce. The gravity of dark matter, concentrated in clusters, magnifies and distorts light from distant background objects. This effect creates distortions in the shapes of the background galaxies that appear in the images of the clusters. The gravitational lens can often also create multiple images of the same distant galaxy.
The higher the concentration of dark matter in a cluster, the more dramatic its bending effect. The presence of smaller rocks of dark matter associated with individual cluster galaxies increases the level of distortion. In a sense, the galactic cluster acts as a large-scale lens with much smaller lenses.
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What did the researchers find?
Hubble’s clear images were taken by the telescope’s wide-range camera 3 and the advanced research camera. Together with the VLT spectra, the team created an accurate map with high accuracy and dark matter. By measuring lens distortions, astronomers can track the amount and distribution of dark matter.
To the team’s surprise, in addition to the dramatic arcs and elongated features of distant galaxies produced by the gravitational lens of each cluster, Hubble images also revealed an unexpected number of smaller scales and distorted images embedded near the core of each cluster. -massive galaxies reside. The researchers believe that the inserted lenses are produced by the gravity of dense concentrations of matter inside individual cluster galaxies. Subsequent spectroscopic observations measure the velocity of stars orbiting several of the cluster galaxies to determine their masses.
By combining Hubble imaging and VLT spectroscopy, astronomers have been able to identify dozens of diverse, objectified background galaxies. This allowed them to compile a well-calibrated map with the distribution of the mass of dark matter in each high-resolution group.
The team compared dark matter maps with samples from simulated clusters of galaxies with similar masses spaced at approximately equal distances. The clusters in the computer model do not show any of the same levels of dark matter concentration on the smallest rocks – the rocks associated with individual cluster galaxies.
“With the help of advanced cosmological simulations, we can compare the quality of the observations analyzed in our article, allowing detailed comparisons like never before,” said Stefano Borgani of the University of Delhi Studi di Trieste, Italy.
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