Dark matter environments may have shaped the internal structure of galaxy clusters
An international teamof researchers claim that the internal structure of vastgalaxy clusters, containing thousands of individual galaxies, may be shaped by the dark matter environments in which they evolve. Thestudy also provides evidence for a theoretical notion known as assembly bias.
Our understanding ofdark matter is still relatively infantile, owing to the fact that itcan only be observed through the influence it asserts over visible matter. Despite this difficulty, breakthroughs in the fieldof astronomy have not only provided strong evidence for itsexistence, but have actually allowed astronomers to estimate thatdark matter makes up around 27 percent of all matter and energy inthe universe.
"Galaxy clustersare remarkable windows into the mysteries of the universe," says Hironao Miyatake of NASA's Jet Propulsion Laboratory, and leadresearcher for the study. "By studying them, we can learn moreabout the evolution of large-scale structure of the universe, and itsearly history, as well as dark matter and dark energy."
For the study, Miyatakeand his team drew on data covering 9,000 of the enormous clusters,collected as part of the Sloan Digital Sky Survey DR8 galaxycatalog. The masses of the clusters were determined by observingthe extent to which the structures warp the light emitted from moredistant objects through a phenomenon known as gravitational lensing.
The researchers dividedthe clusters into two categories based on their internal structure.The first category contained clusters with relatively tightly clumpedstructures, while the galaxies in the others were more sparselydistributed.
According to theresearchers, the dark matter environment in which the clusters evolvehas a significant influence in shaping their internal structures, andthat the root of the phenomenon dates back to the first moments ofthe universe.
In the first trillionthof a second following the Big Bang, mass was first spread across thecosmos via an event known as cosmic inflation. However, mass was notspread evenly due to energy shifts in this formative period known asquantum fluctuations.
These quantumfluctuations are potentially responsible for creating an unevendistribution of matter, resulting in natural peaks of material in thecosmos – described by the researchers as "peaky" regions– that contain a higher than average density of matter.
The current model formass distribution throughout the universe is predicated on Einstein'stheory of general relativity. Astronomers believe that matter is clustered in afilamentary web-like manner – a theory that has been largelysupported by observation.
This model has beenslightly complicated by a notion known as assembly bias, which, atits most basic level. says that the distribution of galaxies andgalaxy clusters are subject to not only the total mass of thestructures, but also the process by which they formed.
In the "peaky"regions of space, galaxy clusters are able to form without the aid ofa dark matter environment. The resultant clusters are believed toform comparatively slowly with the many thousands of galaxies thatform the cluster taking on a densely packed structure
Conversely, Miyatakeand his team discovered that outside of these peaks, galaxy clustersrequire a dark matter-rich environment in order to coalesce. The darkmatter-saturated environment causes the cluster to form significantlyfaster than its "peaky" cousins, albeit with a moresparsely distributed internal structure.
The results of the newstudy highlight a relationship between the dark matter environment inwhich the clusters develop, and their internal structure, while alsoproviding evidence for the notion of assembly bias. However, the biassignal detected by the researchers was significantly higher than thetheoretical expectation.
This could simply be arare exception to the theoretical norm, however it may also indicatea problem with the theory. Further studies will be needed to settlethe discrepancy.
A paper on the studyhas been published online in the online journal Physical Review Letters.