New Observations Suggest Missing Ingredient in Current Dark Matter Theories

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The large mass of a galaxy cluster deflects light from background objects, a phenomenon known as gravitational lensing. The large-scale gravitational lens caused by the whole cluster can be modified by smaller-scale mass concentrations within the cluster, such as individual galaxies. Using the NASA/ESA Hubble Space Telescope and ESO’s Very Large Telescope (VLT), a team of astronomers has examined these small-scale gravitational lenses in 11 galaxy clusters. They’ve found lenses that were an order of magnitude smaller than would be expected from cosmological simulations and concluded that there is an unidentified problem with either prevailing simulation methods or standard cosmology.

This Hubble image shows the massive galaxy cluster MACSJ 1206. Embedded within the cluster are the distorted images of distant background galaxies, seen as arcs and smeared features. These distortions are caused by the dark matter in the cluster, whose gravity bends and magnifies the light from faraway galaxies, an effect called gravitational lensing. This phenomenon allows astronomers to study remote galaxies that would otherwise be too faint to see. Overlaid on the image are small-scale concentrations of dark matter, represented in this artist’s impression in blue. Dark matter is the invisible glue that keeps stars bound together inside a galaxy and makes up the bulk of the matter in the Universe. These blue halos reflect how the galaxy cluster’s dark matter is distributed, revealed by new results from Hubble. Image credit: NASA / ESA / Hubble / G. Caminha, University of Groningen / M. Meneghetti, Observatory of Astrophysics and Space Science of Bologna / P. Natarajan, Yale University / CLASH Team / M. Kornmesser.

This Hubble image shows the massive galaxy cluster MACSJ 1206. Embedded within the cluster are the distorted images of distant background galaxies, seen as arcs and smeared features. These distortions are caused by the dark matter in the cluster, whose gravity bends and magnifies the light from faraway galaxies, an effect called gravitational lensing. This phenomenon allows astronomers to study remote galaxies that would otherwise be too faint to see. Overlaid on the image are small-scale concentrations of dark matter, represented in this artist’s impression in blue. Dark matter is the invisible glue that keeps stars bound together inside a galaxy and makes up the bulk of the matter in the Universe. These blue halos reflect how the galaxy cluster’s dark matter is distributed, revealed by new results from Hubble. Image credit: NASA / ESA / Hubble / G. Caminha, University of Groningen / M. Meneghetti, Observatory of Astrophysics and Space Science of Bologna / P. Natarajan, Yale University / CLASH Team / M. Kornmesser.

“Galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the Universe that are currently available reproduce well what we can infer from gravitational lensing,” said Dr. Massimo Meneghetti, an astronomer at the INAF-Observatory of Astrophysics and Space Science of Bologna.

“We have done a lot of testing of the data in this study, and we are sure that this mismatch indicates that some physical ingredient is missing either from the simulations or from our understanding of the nature of dark matter.”

“There’s a feature of the real Universe that we are simply not capturing in our current theoretical models,” said Dr. Priyamvada Natarajan, an astronomer at Yale University.

“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.”

Hubble images, coupled with spectra from the VLT, helped the researchers produce accurate, high-fidelity dark-matter maps of the 11 galaxy clusters, including MACS J1206.2-0847, MACS J0416.1-2403 and Abell S1063.

To their surprise, in addition to the dramatic arcs and elongated features of distant galaxies produced by each cluster’s gravitational lensing, they also found an unexpected number of smaller-scale arcs and distorted images nested near each cluster’s core, where the most massive galaxies reside.

They believe the nested lenses are produced by the gravity of dense concentrations of matter inside the individual cluster galaxies.

Follow-up spectroscopic observations measured the velocity of the stars orbiting inside several of the cluster galaxies to thereby pin down their masses.

“The data from Hubble and the VLT provided excellent synergy,” said Dr. Piero Rosati, an astronomer at the Università degli Studi di Ferrara.

“We were able to associate the galaxies with each cluster and estimate their distances.”

“The speed of the stars gave us an estimate of each individual galaxy’s mass, including the amount of dark matter,” said Dr. Pietro Bergamini, an astronomer at the INAF-Observatory of Astrophysics and Space Science.

The scientists compared their 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 results of these analyses further demonstrate how observations and numerical simulations go hand in hand,” said Dr. Elena Rasia, an astronomer at the INAF-Astronomical Observatory of Trieste.

“With advanced cosmological simulations, we can match the quality of observations analyzed in our paper, permitting detailed comparisons like never before,” said Dr. Stefano Borgani, an astronomer at the Università degli Studi di Trieste.

The study was published in the journal Science.

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Massimo Meneghetti et al. 2020. An excess of small-scale gravitational lenses observed in galaxy clusters. Science 369 (6509): 1347-1351; doi: 10.1126/science.aax5164

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