Astronomers detect strong shock front in galaxy cluster SPT-CLJ 2031-4037

Using NASA’s Chandra X-ray spacecraft, astronomers from the University of Alabama in Huntsville have observed a merging cluster of galaxies known as SPT-CLJ 2031-4037. They discovered a rarely seen strong shock front in this cluster of galaxies. The finding was reported in a research paper published June 27 on the pre-print server arXiv.

Galaxy clusters are formed by hierarchical mergers of smaller subclusters and contain up to thousands of galaxies that are gravitationally bound to each other. They are the largest known gravitationally bound structures in the universe and could serve as excellent laboratories for studying the evolution and cosmology of galaxies.

Mergers of galaxy clusters are the most energetic events in the Universe after the Big Bang. A fraction of the kinetic energy released during these mergers is carried away into the intracluster medium via shocks and turbulence. The so-called shock fronts, seen as sharp discontinuities in X-ray brightness and temperature, provide astronomers with a rare opportunity to observe and study such merger systems and their geometry.

SPT-CLJ 2031-4037 (or simply SPT J2031) is a merging galaxy cluster with a redshift of 0.34. It is a huge system with an estimated mass of about 800 trillion solar masses and an X-ray luminosity of 1.04 quattuordecillion erg/s.

A team of astronomers led by Purva Diwanji of the University of Alabama used the Chandra X-ray Observatory to search for shock fronts in SPT J2031.

“SPT J2031 was observed by the Chandra Advanced CCD Imaging Spectrometer (ACIS) detector in the Very Faint (VFAINT) mode for a total time of 256 ks, divided into 10 observations,” the researchers wrote in the paper.

The observation allowed the team to detect two shock fronts in SPT J2031: the stronger one to the northwest and the weaker one to the southeast (the southeastern edge). The stronger shock front has a density jump of 3.16 over the sharp surface brightness edge and a Mach number of 3.36, while the weaker one has a density jump of 1.53 and a Mach number of 1.36.

The paper’s authors emphasize that the finding makes SPT J2031 one of the rare merging systems with a Mach number above 2.0. They note that Chandra has discovered only a handful of merging shock fronts with such a high Mach number.

The study also found that SPT J2031 exhibits fusion geometry and that the post-shock electron temperature of the stronger shock front is lower than the temperature predicted by the instant shock heating model and favors the collisional equilibrium model. These findings are consistent with results provided by previous studies.

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