Astronomers observe 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 cluster of merged galaxies known as SPT-CLJ 2031-4037. They discovered a strong shock front rarely seen in this cluster of galaxies. The discovery was reported in a research paper published June 27 on the server ahead of print arXiv.

Galaxy clusters are formed through hierarchical mergers of smaller subgroups and contain up to thousands of galaxies bound together by gravity. They are the largest known gravitationally bound structures in the universe and can serve as excellent laboratories for the study of galactic evolution and cosmology.

Mergers of galaxy clusters are the most energetic events in the universe after the Big Bang. Some of the kinetic energy released during these mergers is dissipated into the environment within the cluster through shocks and turbulence. So-called shock fronts, seen as sharp discontinuities in X-ray luminosity and temperature, give astronomers a rare opportunity to observe and probe such merger systems and their geometry.

SPT-CLJ 2031-4037 (or SPT J2031 for short) is a cluster of merged galaxies at a redshift of 0.34. It is a massive system with an estimated mass of about 800 trillion solar masses and X-ray luminosity at a rate of 1.04 quatuordecillion erg/s.

A team of astronomers led by Purva Diwanji of the University of Alabama has 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 write in the paper.

The observation allowed the team to detect two shock fronts in SPT J2031 – the strongest in the northwest and the weakest in the southeast (southeast edge). The strongest shock front has a density jump of 3.16 along the sharp edge of the surface gloss 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 point out that the finding makes SPT J2031 one of the rare merging systems with a Mach number above 2.0. They note that only a small fraction of merger shock fronts with such high Mach numbers have been detected by Chandra.

The study also found that SPT J2031 exhibits merger geometry and that the electron temperature after impact of the strongest part of the shock is lower than the temperature predicted by the instantaneous shock heating model and favors the collisional equilibrium model. These findings are consistent with the results provided by previous studies.

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