Astronomers observe powerful shock front in galaxy cluster SPT-CLJ 2031-4037

Using NASA’s Chandra X-ray probe, astronomers at the University of Alabama in Huntsville have observed a merging galaxy cluster known as SPT-CLJ 2031-4037. They detected a powerful, rarely observed shock front in the galaxy cluster. The discovery was reported in a research paper published June 27 on the preprint server arXiv.

Galaxy clusters form by hierarchical mergers of smaller subclusters and contain up to thousands of galaxies bound together by gravity. They are the largest known gravitational structures in the universe and could serve as excellent laboratories for studying galaxy evolution and cosmology.

Galaxy cluster mergers are the most energetic events in the Universe after the Big Bang. A fraction of the kinetic energy released during these mergers is dissipated into the intra-cluster medium by shocks and turbulence. Shock fronts, observed as strong discontinuities in X-ray luminosity and temperature, offer astronomers a rare opportunity to observe and study such merging systems and their geometry.

SPT-CLJ 2031-4037 (or SPT J2031 for short) is a merging galaxy cluster with a redshift of 0.34. It is a massive 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 conducted a search for shock fronts in SPT J2031 with the help of the Chandra X-ray Observatory.

“SPT J2031 was observed by the Chandra Advanced CCD Imaging Spectrometer (ACIS) detector in Very Faint (VFAINT) mode for a total of 256 ks spread over 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 (southeast edge). The stronger shock front has a density jump of 3.16 on the sharp edge of the bright surface 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 study authors point out that this discovery makes SPT J2031 one of the few merging systems with a Mach number greater than 2.0. They note that only a handful of merger shock fronts with such a high Mach number have been discovered by Chandra.

The study also revealed that SPT J2031 exhibits a fusion geometry and that the post-shock electron temperature of the strongest shock front is lower than the temperature predicted for the instantaneous shock heating model and favors the collisional equilibration model. These results are consistent with the results provided by previous studies.

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