Does gas in galaxy clusters flow like honey?

We have seen intricate patterns that milk produces in coffee and much smoother than those made by honey when mixed with a spoon. Which of these cases best describes the behavior of hot gas in clusters of galaxies? In answering this question, a new study using NASA's Chandra X-ray observatory deepened our understanding of clusters of galaxies, the largest structures in the universe held together by gravity.

Clusters of galaxies are composed of three main components: individual galaxies, a multi-million-degree gas that fills the gap between galaxies and dark matter, a mysterious form of matter dispersed in a cluster and representing about 80 % of the mass of the cluster.

A team of astronomers used a series of long observations of Chandra, totaling about two weeks of observation, from the cluster of Coma galaxies to probe the properties of gases at comparable spatial scales at the typical distance traveled by the particles between them. This measurement allowed them to better know the viscosity – the technical term designating the resistance to the movement of the balls of gas with respect to each other – of the hot gas of Coma.

"Our discovery suggests that the viscosity of the gas at 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 clusters of galaxies at small scales, analogous to the swirling movements of a cup of coffee."

The hot gas in Coma shines in the X-ray light observed by Chandra. It is known that the gas contains about six times more mass than all the galaxies combined of the group. Despite its abundance, the density of hot gas in Coma, which, according to radio observations, is penetrated by a weak magnetic field, is so small that the particles do not interact very often with each other. Such a low density hot gas can not be studied in a terrestrial laboratory. Scientists must therefore call upon cosmic laboratories such as that provided by Coma's intergalactic gas.

"We used Chandra to determine if the gas density was smooth on the smaller scales that we could detect," said Eugene Churazov, co-author of the Max Planck Institute for Astrophysics of Garching and the Institute of Space Research in Moscow. "We found that this was not the case, suggesting that turbulence is present even on these relatively small scales and that the viscosity is low."

To reach these conclusions, the team focused on a region far from the center of the Coma pole where the density of hot gas is even lower than in the center. Here, particles have to travel longer distances – about 100,000 light-years on average – to interact with another particle. This distance is large enough to be probed with Chandra.

"Perhaps one of the most surprising aspects is that we have been able to study physics at scales that are relevant to the interactions between atomic particles in an object 320 million light-years apart," said the co-ordinator. author Alexander Schekochihin from Oxford University in the United States. Kingdom. "Such observations offer a great opportunity to use clusters of galaxies as laboratories to study the fundamental properties of hot gas."

Why is the viscosity of Coma's hot gas so low? One explanation is the presence of small scale irregularities in the magnetic field of the cluster. These irregularities can deflect the particles in the hot gas, composed of electrically charged particles, mainly electrons and protons. These deviations reduce the distance that a particle can travel freely and, by extension, the viscosity of the gas.

Knowledge of the viscosity of the gas in a group of galaxies and the ease with which turbulence develops helps scientists understand the effects of important phenomena such as collisions and fusions with other groups of galaxies and groups. of galaxies. The turbulence generated by these powerful events can serve as a source of heat, preventing the hot gases of the bunches from cooling to form billions of new stars.

The researchers chose the Coma cluster for this study because it offers the best combination of physical properties required. The average distance between particle collisions is higher for gases with higher temperatures and lower densities. Coma is hotter than other clusters of the brightest galaxies nearby and has a relatively low density, unlike the cold and dense cores of other clusters of bright galaxies, including Perseus and Virgo. This gives astronomers a chance to use the Coma cluster as a laboratory for studying plasma physics.

Future direct measurements of the speed of gas movements 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 conduct in-depth studies of many galaxies near clusters. XRISM is expected to be launched in the early 2020s.

An article describing this result was published in the June 17 issue of the journal Nature Astronomy.