We have seen intricate patterns that milk produces in coffee and much softer than honey produces when mixed with a spoon. Which of these cases best describes the behavior of hot gas in galaxy clusters? Answering this question, a new study using NASA's Chandra X-ray Observatory has deepened our understanding of galaxy clusters, the largest structures of the universe held together by gravity.
Clusters of galaxies are composed of three main components: individual galaxies, a multimillion-dollar gas that fills the space between galaxies and dark matter, a mysterious form of matter that develops in a cluster and represents about 80% of the mass of the bunch.
A group of astronomers used a series of long observations by Chandra, for a total of about two weeks of observation, of the cluster of Coma galaxies to probe the properties of gas on comparable spatial scales with a typical distance that the particles travel between collisions the one with the other. This measure helped them to know the viscosity – the technical term for the resistance to motion of the gas nodules one compared to the other – of the hot gas in Coma.
"Our results suggest that the viscosity of the gas in Coma is much lower than expected," said Irina Zhuravleva of the University of Chicago, who led the study. "This means that turbulence can easily develop in hot gas in clusters of galaxies on small scales, similar to swirling movements in a coffee cup."
Coma's hot gas emits X-ray light observed by Chandra. The gas is known to contain about six times more mass than all the galaxies combined in the cluster. Despite its abundance, the density of the hot gas in Coma, which the radio observations showed is permeated by a weak magnetic field, is so low that the particles do not interact very often with each other. Such a low-density hot gas cannot be studied in a laboratory on Earth, and therefore scientists must rely on cosmic laboratories such as that provided by the intergalactic gas in Coma.
"We used Chandra to probe if the gas density is homogeneous on the smallest scales we can detect," said Eugene Churazov, a co-author of the Max Planck Institute for Astrophysics in Garching and the Space Research Institute in Moscow. "We found that it is not, suggesting that turbulence is also present on these relatively small scales and the viscosity is low."
To reach these conclusions, the team focused on a region far from the center of the Coma Cluster, where the density of hot gas is even lower than in the center. Here, the particles must travel longer distances – about 100,000 light years on average – to interact with another particle. This distance is large enough to be explored with Chandra.
"Perhaps one of the most surprising aspects is that we were able to study physics on scales relevant to interactions between atomic particles in an object 320 million light years away," said co-author Alexander Schekochihin of the University of Oxford in the United States. Kingdom. "Such observations open up a great opportunity to use galaxy clusters as laboratories to study the fundamental properties of hot gas."
Why is Coma's hot gas viscosity so low? An explanation is the presence of small-scale irregularities in the cluster's magnetic field. These irregularities can deflect the particles into the hot gas, which is composed of electrically charged particles, mainly electrons and protons. These deflections reduce the distance that a particle can move freely and, by extension, the viscosity of the gas.
Knowledge of the viscosity of gas in a cluster of galaxies and the ease with which turbulence develops helps scientists to understand the effects of important phenomena such as collisions and fusions with other galaxy clusters and groups of galaxies. The turbulence generated by these powerful events can act as a heat source, preventing the hot gas from the clusters from cooling down to form billions of new stars.
The researchers chose the Coma cluster for this study because it has the best combination of required physical properties. The average distance between particle collisions is greater for gas with higher temperatures and lower densities. The coma is warmer than other clusters of galaxies in the brightest vicinity and has a relatively low density, unlike the cold and dense nuclei of other clusters of brilliant galaxies, including Perseus and Virgo. This gives astronomers the opportunity to use the Coma cluster as a laboratory to study plasma physics.
Future direct measurements of gas motion velocities with the X-ray imaging and spectroscopy (XRISM) mission, a collaborative mission between the Japanese exploration agency and NASA, will provide more details on cluster dynamics, allowing us to do robust studies on many nearby cluster galaxies. XRISM is expected to be launched in the early 2020s.
An article describing this result appeared in the June 17 issue of the magazine Natural astronomy.
Clues to the growth of the colossus in Coma
I. Zhuravleva et al. Effective viscosity suppressed in intergalactic bulk plasma Natural astronomy (2019). DOI: 10.1038 / s41550-019-0794-z , https://arxiv.org/abs/1906.06346
Does gas in galaxy clusters flow like honey? (2019, June 18)
recovered on June 18, 2019
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