Findings by the Hubble Space Telescope and the Very Large Telescope (VLT) have shown that there might be something lacking in the theories of how dark matter is behaving.
This missing component may explain why researchers have discovered an unexplained inconsistency between measurements of concentrations of dark matter in a study of large galaxy clusters, and theoretical, computational models of dark matter are spread in clusters. The latest observation suggests that certain small-scale amounts of dark matter yield 10 times greater lensing effects than predicted.
Dark matter is the intangible glue that holds together stars, dust, and gas in a galaxy. This unknown material represents the bulk of the mass of a galaxy, which forms the basis of the large-scale structure of our Universe. Since dark matter does not emit, absorb, or mirror light, its presence is understood in space only by its gravitational force on visible matter. Astronomers and physicists strive to work to track down what it is.
Galaxy clusters are the largest reservoirs of dark matter. Clusters are composed of individual component galaxies, which are essentially bound together by dark matter gravity.
The distribution of dark matter in clusters is mapped by calculating the bending of light, which they create - the gravitational lensing effect. The mass of dark matter contained in clusters magnifies and distorts light from distant sources in the background. This influence creates irregularities in the structures of the surrounding galaxies that appear in cluster pictures. Often times, gravitational lensing will create multiple images of the same distant galaxy.
The higher a cluster's concentration of dark matter, the more intense the light-bending effect is. The presence of tiny clumps of dark matter connected to individual cluster galaxies raises the number of distortions. The galaxy cluster, in some ways, functions like a large-scale lens with several smaller lenses located within it.
The images from Hubble were taken by the observatory's Wide Field Camera 3 and Advanced Camera for Surveys. The team was able to produce a detailed map of dark matter with the help of spectra from the European Southern Observatory's VLT.
Scientists could trace the amount and distribution of dark matter by calculating the lensing distortions. Part of two Hubble surveys, The Frontier Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH) programs, were three key galaxy clusters: MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063.
To the surprise of the team, besides the spectacular arcs and elongated features of distant galaxies created by the gravitational lensing of each cluster, the Hubble images also showed a surprising number of smaller arcs and distorted images snuggled near the core of each cluster, where the largest galaxies reside. The researchers believe that the entangled lenses within the individual cluster galaxies are created by the gravity of dense concentrations of matter. Follow-up spectroscopic experiments measured the stars orbiting within some of the cluster galaxies to pin down their masses.
The astronomers were able to identify hundreds of massively imaged, lensed, background galaxies by integrating Hubble imagery and VLT spectroscopy. This helped them to assemble, in each cluster, a well-calibrated, high-resolution map of the mass distribution of dark matter.
The team compared the maps of dark matter with observations of simulated galaxy clusters of equal masses at about the same distances. The clusters in the computer model did not exhibit any of the smallest scales at the same degree at dark-matter concentration - the scales correlated with individual cluster galaxies.
The research is described in a paper published in the journal Science.
