More small-scale dark matter gravitational lenses than expected in galaxy clusters

Katie Ramirez
September 14, 2020

This finding surprised astronomers because it differs from their theoretical models about the distribution of dark matter in galaxy clusters. Features of the Cosmic Microwave Background can be explained by the presence of dark matter. These simulations are then tested against what they see in more observations, to ensure they are correctly emulating reality.

The higher the concentration of dark matter in a cluster, the more dramatic its light-bending effect.

Scientists know that because of dark matter's gravitational influence on the other things that are visible, like clusters of galaxies. This missing stuff is called dark matter, and, despite constituting the vast majority of matter in the universe, it doesn't emit, absorb, or reflect light. Through gravitational interactions with itself, the dark matter formed intersecting fibers in a complex, three-dimensional meshwork.

Among the large-scale distortions the astronomers were expecting to find, they also spotted smaller areas of warping, which they suspect mark the locations of individual, smaller cluster galaxies that hide concentrations of dark matter.

So, objects on the far side of that gravitational field, such as distant galaxies, appear to us magnified, smeared, duplicated, and distorted. This effect, called a gravitational lens, allows astronomers to study distant galaxies that otherwise would be too faint to be seen. If a massive object-say, a galaxy-sits between us and a distant object, it can create a gravitational lens that magnifies or distorts the distant object. Galaxies in the three withdrawals are examples of these effects. The red dots around the galaxy in the upper left indicate an emission from hydrogen clouds at one distant source. These points were detected by the Multi-Unit Spectral Explorer (MUSE) in the Very Large Telescope (VLT) at the European Southern Observatory in Chile. The blobs do not appear in the Hubble images. The Hubble image is a combination of visible and infrared light observations captured in 2011 by the Advanced Surveys Camera and Wide Field Camera 3. Likewise, the larger the cluster of dark matter - the greater the mass - the more extreme the curvature of space and therefore, light. Clusters are composed of individual member galaxies that are held together largely by the gravity of dark matter.

"Galactic clusters are ideal laboratories for understanding whether computer simulations of the universe reproduce what we can infer about dark matter and its interaction with luminous matter", said Massimo Mingetti of the National Institute of Astrophysics - Observatory of Astrophysics and Space Sciences.

"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.

Researchers suggest that there are two explanations for this discrepancy: we do not appreciate all the properties of the dark matter, nor do we find any in the simulations of the evolution of the universe.

"There's a feature of the real universe that we are simply not capturing in our current theoretical models", said Priyamvada Natarajan, senior theorist on the research team and theoretical astrophysicist at Yale University, in a statement.

Dark matter is not behaving as we would expected it to, scientists have said.

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The study published Friday in the journal Science. Impatient theoretical cosmologists will undoubtedly test dark matter transformations long before further reanalysis emerges. Using this map, scientists traced lensing distortions by examining how matter distorts light and mapping dark matter in clusters with high accuracy.

Hubble's crisp images, coupled with spectra from the VLT, helped the team produce an accurate, high-fidelity dark-matter map.

By combining Hubble imaging and VLT spectroscopy, the astronomers were able to identify dozens of multiply imaged, lensed, background galaxies. These small scale clumps of dark matter enhance the overall distortion.

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. But if we were to adjust our models to make these galaxies more diffuse, then we'd be less likely to see more compact structures in galaxy clusters.

The distribution of dark matter can be viewed as a prediction of the model. Just like a fun mirror, the dark matter's gravity amplifies and distorts light from distant background objects, producing distortions and sometimes multiple images of the same distant galaxy.

Follow-up spectroscopic observations added to the study by measuring the velocity of the stars orbiting inside several of the cluster galaxies.

"The data from Hubble and the VLT provided excellent synergy", says team member Piero Rosati, Università Degli Studi di Ferrara in Italy. The hypothesized matter is thought to exist based on the mass of galaxies, but has never been directly observed.

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 model did not show any of the same level of dark-matter concentration on the smallest scales - the scales associated with individual cluster galaxies.

Astronomers, including those of this team, look forward to continuing to probe dark matter and its mysteries in order to finally pin down its nature.

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