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

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

Clusters of galaxies are formed by the hierarchical merging of smaller subclusters and contain up to thousands of galaxies bound together by gravity. They are the largest known gravitationally bound structures in the universe and could serve as excellent laboratories for the study of galaxy evolution and cosmology.

Galaxy cluster mergers are the most energetic events in the universe since the Big Bang. A fraction of the kinetic energy released during these mergers is dissipated into the intracluster medium through shocks and turbulence. So-called shock fronts, seen as sharp discontinuities in X-ray brightness and temperature, give 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 at 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 pieces spread over 10 observations,” the researchers wrote in the paper.

The observations allowed the team to detect two shock fronts in SPT J2031 – stronger in the northwest and weaker in the southeast (southeastern edge). The stronger shock front has a density jump of 3.16 across the surface brightness sharp 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 authors of the paper emphasize that this finding makes SPT J2031 one of the few merging systems with a Mach number higher than 2.0. They note that Chandra has discovered only a handful of merging shock fronts with such high Mach numbers.

The study also found that SPT J2031 exhibits a merger geometry and that the post-shock electron temperature of the stronger shock front is lower than that predicted by the instantaneous shock heating model, favoring the collisional equilibration model. These findings are consistent with the results provided by previous studies.

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