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.
“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.”
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.