New Aspect of Mysterious Dark Matter in Universe Revealed

New Hubble data suggests there is an ingredient missing from current dark matter theories

When the team sat down to analyse the data, they found the large-scale lensing effects as expected to be produced by the galaxy as a whole. If a huge object (such as a galaxy) is between us and a distant object, we can create a gravitational lens that magnifies or distorts the distant object. The red blobs around the galaxy at upper left denote emission from clouds of hydrogen in a single distant source. This phenomenon allows astronomers to study remote galaxies that would otherwise be too faint to see. These points were detected by the Multi-Unit Spectral Explorer (MUSE) in the Very Large Telescope (VLT) at the European Southern Observatory in Chile. Dots do not appear in Hubble images. This allowed researchers to determine which objects were behind the galaxy cluster, thus potential candidates for gravitational lensing. 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. Those behind the study suggest that something is wrong with the model, or that our understanding of the dark matter may need adjustment. These small lenses, produced by individual galaxies within the clusters, did not appear in simulations of the clusters, suggesting an excess of dark matter. They used follow-up spectroscopic observations to measure the velocity of the stars within these clusters and through a calculation method known as viral theorem, confirmed the masses of these clusters, and in turn, the amount of dark matter they contain. Over time, the continuous equilibrium of gravity pulled the galaxies together, forming large clusters.

Dark matter remains nearly entirely elusive. The inset shows the derived detailed spatial distribution of dark matter - the heaped mountain range dotted with small peaks.

The team of researchers is planning on gathering more data from the galaxy clusters in order to investigate their results further, and find out why it does not match up with current simulations.

The full nature of dark matter remains elusive, but new observations of a distant galaxy cluster have revealed that something is missing from even the fundamental theories astronomers use to map this invisible part of the universe.

The discovery of this incompatibility could help astronomers design better computer simulation models so they can better understand what dark matter clusters are, ultimately discovering what this abundant and dominant form of matter really is. This is the case when the distribution of dark matter is slightly more bumpy than the model predicts.

"Galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the Universe that are now available reproduce well what we can infer from gravitational lensing", said Massimo Meneghetti of the INAF-Observatory of Astrophysics and Space Science of Bologna in Italy, the study's lead author. From Bologna, Italy, lead author of the study.

"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", added Meneghetti.

Our understanding of dark matter and its behavior could be missing a key ingredient.

"What we learned from this result is that there is something of the real universe that our current theoretical models can not reproduce", Meneghetti says.

Dark matter is not behaving as we would expected it to, scientists have said. Dark on the smallest scales.

The research is published this month in the journal Science.

Researchers spotted some small lenses, generated by individual galaxies within the clusters. The higher the concentration of dark matter in a cluster, the more dramatic its light bending.

This image overlays dark matter distribution determined by a simulation based on data from the ESO's Very Large Telescope.

By combining Hubble imaging and VLT spectroscopy, the astronomers were able to identify dozens of multiply imaged, lensed, background galaxies.

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.

Hubble Space Telescope image of the lensing cluster MACSJ1206.

The lensing that the team believes accounts for dark matter discrepancies is a factor of Einstein's theory of general relativity which suggests that gravity is actually an effect that mass has on spacetime. These distortions are caused by the amount of dark matter in the cluster, whose gravity bends and magnifies the light from faraway galaxies. Before astronomers can figure this one out, it's going to take new observations of galaxy clusters, to potentially find more of these discrepancies. It also creates an invisible scaffold that enables galaxies to form clusters.

"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.

The team looks forward to continuing their stress-testing of the standard dark-matter model to pin down its intriguing nature.

NASA's planned Nancy Grace Roman Space Telescope will detect even more remote galaxies through gravitational lensing by massive galaxy clusters.



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