Free Astronomy Magazine January-February 2021

27 JANUARY-FEBRUARY 2021 ASTRO PUBLISHING T his video shows an artist’s im- pression of the phenomenon of gravitational lensing. [ESA/Hubble and M. Kornmesser] derstanding 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.” The distribution of dark matter in clusters is mapped by measuring the bending of light — the gravita- tional lensing effect — that they produce. The gravity of dark matter concentrated in clusters magnifies and warps light from distant back- ground objects. This effect pro- duces distortions in the shapes of background galaxies which appear in images of the clusters. Gravita- tional lensing can often also pro- duce multiple images of the same distant galaxy. The higher the concentration of dark matter in a cluster, the more dramatic its light-bending effect. The presence of smaller-scale clumps of dark matter associated with individual cluster galaxies en- hances the level of distortions. In some sense, the galaxy cluster acts as a large-scale lens that has many smaller lenses embedded within it. Hubble’s crisp images were taken by the telescope’s Wide Field Cam- era 3 and Advanced Camera for Sur- veys. Coupled with spectra from the European Southern Observatory’s Very Large Telescope (VLT), the team produced an accurate, high- fidelity, dark-matter map. By measuring the lensing distor- tions astronomers could trace out the amount and distribution of dark matter. The three key galaxy clusters, 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, in addition to the dramatic arcs and elongated features of distant galaxies pro- duced by each cluster’s gravita- tional lensing, the Hubble images also revealed an unexpected num- ber of smaller-scale arcs and dis- torted images nested near each cluster’s core, where the most mas- sive galaxies reside. The researchers believe the nested lenses are pro- duced by the gravity of dense con- centrations of matter inside the individual cluster galaxies. Follow-up spectroscopic observa- tions measured the velocity of the stars orbiting inside several of the cluster galaxies to therby pin down their masses. “The data from Hubble and the VLT provided excellent synergy,” shared team member Piero Rosati of the Università degli Studi di Ferrara in Italy, who led the spectroscopic campaign. “We were able to associ- ate 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,” added team member Pietro Bergamini of the INAF-Observatory of Astrophysics and Space Science in Bologna, Italy. By combining Hubble imaging and VLT spectroscopy, the astronomers were able to identify dozens of mul- tiply imaged, lensed, background galaxies. This allowed them to as- semble a well-calibrated, high-reso- lution map of the mass distribution of dark matter in each cluster. The team compared the dark-mat- ter maps with samples of simulated galaxy clusters with similar masses, located at roughly the same dis- tances. The clusters in the computer model did not show any of the same level of dark-matter concen- tration on the smallest scales — the scales associated with individual cluster galaxies. “The results of these analyses further demonstrate how observations and numerical simulations go hand in hand” , said team member Elena Rasia of the INAF-Astronomical Observatory of Trieste, Italy. “With advanced cosmological simu- lations, we can match the quality of observations analysed in our paper, permitting detailed comparisons like never before,” added Stefano Borgani of the Università degli Studi di Trieste, Italy. Astronomers, including those of this team, look forward to continu- ing to probe dark matter and its mysteries in order to finally pin down its nature. !