The intracluster medium becomes extreme hot instantly by shocks induced by cluster mergers

Released: 24th Sep., 2024, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan

Banner Image Credit: X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.

Galaxy clusters continue to grow in mass through mergers with other galaxy clusters. High-energy phenomena induced by cluster mergers, such as shocks, play a significant role in the thermal evolution of diffuse, hot X-ray-emitting gas, known as the intracluster medium (ICM). For example, the ICM is heated by shocks, increasing its gas temperature from around 10 million to a few hundred million Kelvin. However, the heating mechanisms by shocks are unclear. 

In general, two types of shock-heating mechanisms have been proposed: instant-equilibration and adiabatic compression. The instant-equilibration model predicts that the electrons rapidly equilibrate with the ions, while the adiabatic compression model predicts that an adiabatic compression of the electron population at the shock and a subsequent slower equilibration with the ions. In the case of shock-heating for the interstellar medium (ISM) by, e.g., shocks by supernova explosions, the instant-equilibration model is considered to be a promising mechanism. However, the Mach number (M) of shocks by supernova explosions is typically M >> 10, while that by cluster mergers is M ~ 1 - 3, implying that it is unclear whether or not we can assume the same shock-heating mechanisms to the ICM as the case of the ISM. To distinguish between these two models, accurate measurements of the three-dimensional (3D) temperature profiles of the ICM in the post-shock region are important.

NSTC Research Scholar Dr. Shutaro Ueda at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan and Dr. Yuto Ichinohe at RIKEN Nishina Center for Accelerator-Based Science in Japan, analyze two shock fronts in merging galaxy clusters observed with the Chandra X-ray Observatory using a forward-modeling approach. This approach allows simultaneous measurement of the 3D thermodynamic structures of the ICM and reveals that (1) the ICM temperature is highest at the shock front, and (2) the ICM temperature appears isothermal with a temperature of ∼ 10 keV until around 300 kpc away from the shock front in the post-shock region. These findings are consistent with the hypothesis of the instant-equilibration model. 

Although further evidence is still needed to conclude that the instant-equilibration model is a promising mechanism for shock-heating, this forward-modeling approach is useful to explore the 3D thermodynamic structures of the ICM across shock fronts. These results also have significant implications for upcoming X-ray observations such as the new-generation X-ray observatory, XRISM, and the observations of Sunyaev-Zel’dovich effect from the Atacama Large Millimeter/submillimeter Array (ALMA).

More Information:

This research presented in a paper “Three-dimensional thermodynamic structures of the intracluster medium across edges in the X-ray surface brightness of massive, bright, dynamically-active galaxy clusters,” by Shutaro Ueda and Yuto Ichinohe has appeared in the Astrophysical Journal on Sep. 23, 2024.

Media Contact:

Dr. Shutaro Ueda, Email: sueda@asiaa.sinica.edu.tw, Tel: +886 2 2366 5471