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Using NASA's Chandra X-ray observatory, astronomers have observed a massive and hot galaxy cluster known as PLCKG287.0+32.9 (or PLCKG287 for short). Results of the observational campaign, March 17 on the arXiv pre-print server, deliver important insights into the morphological and thermodynamical properties of this cluster.

Galaxy clusters contain up to thousands of galaxies bound together by gravity. They generally form as a result of mergers and grow by accreting sub-clusters. These processes provide an excellent opportunity to study matter in conditions that cannot be explored in laboratories on Earth. In particular, merging galaxy clusters could help us better understand the physics of shock and cold fronts seen in the diffuse intra-cluster medium, the cosmic ray acceleration in clusters, and the self-interaction properties of dark matter.

PLCKG287, also known as PSZ2 G286.98+32.90, is a galaxy cluster at a redshift of 0.38, with a mass of about 1.37 quadrillion solar masses and a temperature of 13 keV. The cluster has an X-ray luminosity in the 2–10 keV band at a level of 1.7 quattuordecillion erg/s.

Previous observations of PLCKG287 have found that it hosts two symmetrically radio relics, which are supposed to be generated by merger-driven shock fronts, and a radio halo, which is connected to re-acceleration of cosmic ray electrons by turbulence. Moreover, the cluster is undergoing one or multiple mergers with the main axis oriented in the direction from northwest to southeast.

A team of astronomers led by Myriam Gitti of the University of Bologna in Italy decided to take a closer look at PLCKG287 due to its peculiar properties. They employed Chandra's Advanced CCD Imaging Spectrometer (ACIS) to perform a detailed analysis of this cluster in X-rays.

The observations found that PLCKG287 showcases a comet-like X-ray morphology oriented in the northwest-southeast direction. The cluster appears to have a bright X-ray core region which is co-spatial with the known giant radio halo. The X-ray peak is spatially offset by about 260,000 light years from the main brightest cluster galaxy (BCG), which suggests a disturbed dynamical state.

By analyzing the central region of PLCKG287 (within 4.9 million light years of the center), the astronomers found that the cluster has a global temperature of approximately 12.73 keV and its X-ray luminosity (in the 2–10 keV band) is 1.09 quattuordecillion erg/s. The integrated gas mass of PLCKG287 was calculated to be about 0.16 quadrillion solar masses, while its total mass was estimated to be 1.2 quadrillion solar masses.

Furthermore, the observations detected a shock front at a distance of about 1.27 million light years northwest from the cluster center. This shock front is characterized by a density jump of 1.43, Mach number of 1.29 and velocity of approximately 2,100 km/s. A cold front was also detected in the same direction, some 962,000 light years from the cluster center, with a density jump of 1.44.

Therefore, based on the results, the authors of the paper assume that PLCKG287 hosts a former cool core and a heating event has increased the temperature of the cluster central region. They concluded that the combined action of merger and central active galactic nucleus (AGN) feedback may explain the reheated cool core, the large-scale shock and the cold front in PLCKG287.

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