Embedded within the cluster are the distorted images of distant background galaxies, which are seen as stained arcs and features. These distortions are caused by the amount of dark matter in the cluster, whose gravity bends and amplifies light from distant galaxies. This effect, called gravitational lensing, allows astronomers to study remote galaxies that would otherwise be too faint to see. Many cluster galaxies are massive and dense enough to distort and amplify distant sources. Gravitational lensing can often also produce multiple images of the same distant galaxy. The red blobs around the galaxy at upper left denote emission from clouds of hydrogen in a single distant source. If a galaxy is hovering above in another dimension, we would not be able to see it. Dots do not appear in Hubble images. The Hubble image is a combination of visible- and infrared-light observations taken in 2011 by the Advanced Camera for Surveys and Wide Field Camera 3.
Image credit: NASA, European Space Agency, b.
Dark matter does not emit, absorb, or reflect light.
"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", he explained. However, it does interact with the Universe's visible matter via gravity.
"One new theory says that dark matter may be ordinary matter in a parallel universe". Overlaid on the image are small-scale concentrations of dark matter, represented in this artist's impression in blue. 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.
Astronomers seem to have uncovered baffling details of the way dark matter behaves. They found small, dense concentrations of dark matter bend and amplified the light with a much greater force than expected.
Galaxy clusters, the largest structures in the universe made up of individual galaxies, are the largest reservoirs of dark matter. Not only are they closely related to each other by the gravitational pull of dark matter, the individual cluster galaxies themselves are also filled with dark matter. Thus, dark matter is distributed in groups of both large and small scales.
"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. From Bologna, Italy, lead author of the study.
"We did a lot of rigorous tests comparing the simulations and the data in this study, and we continued to discover the mismatch", Mengiti continued. "One possible reason for this discrepancy is that we might miss some basic physics in the simulations".
"There's a feature of the real Universe that we are simply not capturing in our current theoretical models", added Priyamvada Natarajan of Yale University in Connecticut, USA, one of the senior theorists on the team. This may indicate a gap in our current understanding of the nature and properties of dark matter, as this impressive data allowed us to probe the detailed distribution of matter. Dark on the smallest scales.
The team paper will appear in the 9/11 issue of the magazine Science.
The distribution of dark matter in clusters is mapped by measuring the bending of light - the gravitational lensing effect - that they produce. The higher the concentration of dark matter in a cluster, the more dramatic its light bending.
The clear Hubble images, along with spectra from the VLT, helped the team produce an accurate, high-resolution map of dark matter.
By combining Hubble imaging and VLT spectroscopy, the astronomers were able to identify dozens of multiply imaged, lensed, background galaxies.
One of the ways we can indirectly "detect" dark matter is through gravitational lensing.
The three key galaxy clusters used in the analysis, 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.
This Hubble image shows the massive galaxy cluster MACSJ 1206.
They believe the nested lenses are produced by the gravity of dense concentrations of matter inside the individual cluster galaxies. Dark matter's distribution in the inner regions of individual galaxies is known to enhance the cluster's overall lensing effect.
Follow-up spectroscopic observations added to the study by measuring the velocity of the stars orbiting inside several of the cluster galaxies.
"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. 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.
The team compared the dark-matter maps with samples of simulated galaxy clusters with similar masses, located at roughly the same distances. The clusters in the computer model did not show any of the same level of dark-matter concentration on the smallest scales - the scales associated with individual cluster galaxies. In some sense, the galaxy cluster acts as a large-scale lens that has many smaller lenses embedded within it. The observations will magnify the sample swarms that astronomers can analyze for further testing of dark matter models.
The global team of astronomers in this study consists of M. Meneghetti, G. Davoli, P. Bergamini, P. Rosati, P. Natarajan, C. Giocoli, G. B. Caminha, R. B. Metcalf, E. Rasia, S. Borgani, F. Calura, C. Grillo, A. Mercurio, and E. Vanzella.
The Hubble Space Telescope is a project of worldwide cooperation between NASA and the European Space Agency (European Space Agency). The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations.