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Curated by RSF Research Staff

Unexpected gas dynamic in merging clusters of galaxies

Galaxies use to cluster together due to their mutual gravity forming clusters that are several millions of light years across. Some clusters have only a handful of galaxies (poor clusters) and other clusters with hundreds to thousands of galaxies are called rich clusters. Our Milky Way is part of a poor cluster called the Local Group formed by two large spirals about 3 million light years large, with the Milky Way and Andromeda Galaxy, dominating the two ends. Each large spiral has several smaller galaxies orbiting them.

Clusters of galaxies are still growing through mergers and the energy involved is pretty huge. Strong shocks, rapid rotation, and turbulence are produced in the gas. The dynamics of merging clusters can be studied using an idealized problem: the collision of two spherical, self-gravitating clusters. One of this interesting event can be seen in Abell 1033, a system located about 1.6 billion light years from Earth. In this region, astronomers have found evidence for a faded electron cloud “coming back to life,” much like the mythical phoenix, after two galaxy clusters collided.

Streams of high-energy electrons filled a region hundreds of thousands of light years across and produced a cloud of bright radio emission. This cloud faded over a period of millions of years as the electrons lost energy and the cloud expanded. However, the radio phoenix emerged when another cluster of galaxies slammed into the original cluster, sending shock waves through the system.

Spectral index map of the radio emission in Abell 1033.

Data shows hot gas in the clusters, which seems to have caused the re-ignition of radio emission in the system. The peak of the X-ray emission is seen at the “bottom” of the cluster. Astronomers think they are seeing the radio phoenix soon after it had reborn, since these sources fade very quickly when located close to the center of the cluster. Because of the intense density, pressure, and magnetic fields near the center of Abell 1033, the burst is only expected to last a few tens of millions of years.

In a recent paper describing these observations, the authors talked about the role of nonthermal components (for example, relativistic particles and magnetic fields) in galaxy clusters. Their action is poorly understood because of observational and theoretical difficulties in studying these plasmas on large scales. Our understanding of the nonthermal part of the intracluster medium remains incomplete. The team led by Francesco de Gasperin have identified a phenomenon that can be unveiled only at extremely low radio frequencies and offers new insights into the nonthermal component. They proposed that the interplay between radio-emitting plasma and the perturbed intracluster medium can gently reenergize relativistic particles initially injected by active galactic nuclei. Sources powered through this mechanism can maintain electrons at higher energies than radiative aging would allow. If this mechanism is common for aged plasma, a population of mildly relativistic electrons can be accumulated inside galaxy clusters providing the seed population for merger-induced reacceleration mechanisms on larger scales such as turbulence and shock waves.

"This was totally unexpected. As these clouds of electrons radiate away their energy over time, they should become fainter and disappear. Instead, in this case, after more than a hundred million years, the tail of electrons is brightly glowing. […] It's like being among the last explorers. As soon as we move into uncharted territories, or in this case, unexplored frequencies, our universe is still full of surprises. And this is just a first step. Much is still to be done to understand the complexity of galaxy clusters, and find what is lurking at low radio frequencies."

Francesco de Gasperin, Leiden Observatory, Netherlands

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