7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Shock Waves in the Hot Plasma of Galaxy Clusters

D. Ryu1, H. Kang2, and J.-H. Ha1
1
Department of physics, UNIST (Ulsan National Institute of Science and Technology)2
Department of Earth Sciences, Pusan National University
e-mail (speaker): dsryu@unist.ac.kr
The hot plasma, called the intracluster
medium (ICM), fills the space between
galaxies in galaxy clusters. It possesses thelargest value of the plasma parameter in theuniverse, and its nature has started to be
uncovered through observations and theoreticalwork including simulations. In this hot ICM
plasma. two distinctive types of shock waves
are expected to be induced.
The first category comprises merger shocks,
which arise as a consequence of the mergers ofsubclumps. These shocks have sonic Mach
numbers of Ms ~ a few and appear at theoutskirts of galaxy clusters. The merger shocks
have been observed in X-ray and radio and are
named “radio relics”. Despite weak shocks,
they are capable of injecting particles into thediffusive shock acceleration (DSA) process,aided by various kinetic processes, such as theBell instability for the ion injection [1] and theAlfven ion cyclotron instability for the electron
injection [2], and hence accelerating theseparticles to high energies.
The second category includes accretion
shocks that form due to supersonic gas infall
from filaments and voids in the cosmic web.
These shocks appear in the periphery of galaxy
clusters and are characterized by large sonic
Mach numbers of Ms ~ 10 - 100 owing to therelatively low temperature of the infalling gas.
In the accretion shocks, ions and electrons areexpected to be injected into DSA via astochastic process mediated by the magnetic
field generated through the ion Weibel
instability [3].
This talk mainly focuses on the kinetic
plasma processes operating at the ICM shocks.
Specifically, we present what we have learned
so far through particle-in-cell (PIC) simulations
and theoretical considerations. We alsocomment on the remaining outstanding issues
that still need to be addressed.

References[1] J.-H. Ha, D. Ryu, H. Kang, & A. J. vanMarle, The Astrophysical Journal, 864, 105(2018)[2] J.-H. Ha, S. Kim, D. Ryu, & H. Kang, The
Astrophysical Journal, 915, 18 (2021)[3] J.-H. Ha, D. Ryu, & H. Kang, The
Astrophysical Journal, 944, 199 (2023)

Figure 1. Characteristic Structure of a quasiperpendicular merger shock in a 2D PIC
simulation. The background is the ion numberdensity. The gyromotion of reflected ions
(green circular arrows) generates theovershoot/undershoot structure in the shocktransition, while the backstreaming ofSDA-reflected electrons (magenta cone)
induces the temperature anisotropy and the EFI
in the preshock region. The colored arrows
indicate the regions where DSA (cyan), SSDA
(dark green), and Fermi-like acceleration (lightgreen) operate. The labels for the three
instabilities, AIC, whistler, and EFI, are placedin the regions where the respective instabilities
are excited. During a SDA cycle, electrons drift
in the negative z-direction (into the paper here)
anti-parallel to the convection electric field.
(Ha et al. 2021)