In a new article in the journal Science, a research team led by Massimo Meneghetti has determined that there are dense clumps of dark matter associated with some galaxies in clusters. This was a two-step observation: the Very Large Telescope (VLT) was used to confirm galaxies belonged to the cluster and to measure the velocities of stars in those galaxies – a way to determine the galaxies’ masses using orbital mechanics. They also used the Hubble Space Telescope to look at how the light of distant objects was magnified and sometimes distorted and replicated by the overall gravitational pull of the cluster, and by the individual tugs of member galaxies. 

To quote the press release: To the team’s surprise, in addition to the dramatic arcs and elongated features of distant galaxies produced by each cluster’s gravitational lensing, the Hubble images also revealed an unexpected number of smaller-scale arcs and distorted images nested near each cluster’s core, where the most massive galaxies reside. The researchers believe the nested lenses are produced by the gravity of dense concentrations of matter inside the individual cluster galaxies.

These dense concentrations are not part of our current models of the universe. According to Beneghetti: 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.

Co-author Priyamvada Natarajan goes on to add: 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.

I have no doubt that we will quickly have multiple, overlapping theories to explain what’s going on. For now though, we have a delightful mystery. I, for one, look forward to seeing if these results can be replicated from studies of isolated galaxies. It would be really cool to know if these weird distributions only exist in clusters. Here is to hoping the team is already looking.

More Information

European Space Agency press release 

NASA Hubblesite press release 

Yale University news article 

“An Excess of Small-Scale Gravitational Lenses Observed in Galaxy Clusters,” Massimo Meneghetti et al., 2020 Sep. 10, Science