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.
Astronomers seem to have revealed a puzzling detail in the way dark matter behaves. They found small, dense concentrations of dark matter that bend and magnify light much more strongly than expected.
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 unprecedentedly 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.
distant swirls of galaxies against the black backdrop of space
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.
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.
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 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.