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Using NASA's Chandra and ESA's XMM-Newton spacecraft, an international team of astronomers have performed X-ray observations of a nearby low-mass galaxy cluster designated PSZ2 G181.06+48.47. The observational campaign, detailed in a paper Jan. 13 on the pre-print server arXiv, uncovers essential information regarding the nature and properties of this cluster.

Galaxy clusters contain up to thousands of galaxies bound together by gravity. They form through accretion of mass and infall of smaller sub-structures and are the largest known gravitationally-bound structures in the universe. Therefore, they could serve as excellent laboratories for studying galaxy evolution and cosmology.

With a mass of about 420 trillion solar masses, PSZ2 G181.06+48.47 is a low-mass galaxy cluster at a redshift of 0.24. It has a radius of approximately 3.45 million light years.

Previous observations have found that PSZ2 G181.06+48.47 hosts two giant radio relics (diffuse, elongated radio sources of synchrotron origin) located over 3.9 million light years from the cluster center and a candidate radio halo (enormous regions of diffuse radio emission showcasing a regular morphology).

Given that PSZ2 G181.06+48.47 coincides with the location of diffuse radio sources, it strongly suggests a merging system. However, the detailed merger scenario of this cluster still remains unknown.

That is why a group of astronomers led by Andra Stroe of the Harvard–Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, decided to conduct multiwavelength observations of PSZ2 G181.06+48.47, aiming to investigate its merging process and properties. In the arXiv paper, they present results from the X-ray observational campaign.

"We aim to paint a comprehensive picture of PSZ2 G181.06+48.47 through multiwavelength observations spanning the electromagnetic spectrum. In this first paper of the series, we utilize Chandra and XMM-Newton to unambiguously confirm PSZ2 G181.06+48.47 as a merging cluster, measure its detailed properties and characterize its substructure," the researchers write.

The observations found that PSZ2 G181.06+48.47 is less massive than previously thought as its mass was measured to be 257 trillion solar masses. This makes it one of the lowest mass galaxy clusters ever discovered to host double radio relics.

PSZ2 G181.06+48.47 was found to be cool as its global temperature turned out to be about 3.86 keV. Therefore, it has comparatively a lower temperature than samples of clusters studied with XMM-Newton observations. The results show that the temperatures seem to increase from the northern to the southern core of PSZ2 G181.06+48.47, from a low of 3.2 keV to a peak of 5.5 keV in the southern region of the south core.

The study found that PSZ2 G181.06+48.47 has an X-ray morphology elongated along the northeast-southwest direction, indicating a cluster merger process is under way along this axis. In particular, the cluster displays two X-ray peaks separated by about 1.2 million light years and connected through a bridge of emission, likely tracing two distinct subclusters observed post-core passage. Both subclusters are compact, with the southern subcluster being brighter.

Based on the collected data, the astronomers conclude that PSZ2 G181.06+48.47 is a very disturbed galaxy cluster with a complex morphological and thermodynamic structure. Its radio relics have the widest separation of all known double-relic clusters.

The obtained results also provide more insights into the merging process of PSZ2 G181.06+48.47.

"The existing observational evidence points to a late-stage, post-merger scenario with a significant LOS [line of sight] component, where the two subclusters are seen post-apocenter as they are falling back into each other," the authors conclude.

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