Turbulence Dynamo in the Stratified Medium of Galaxy Clusters

Abstract

The existence of microgauss magnetic fields in galaxy clusters have been established through observations of synchrotron radiation and Faraday rotation. They are conjectured to be generated via small-scale dynamo by turbulent flow motions in the intracluster medium (ICM). Some of giant radio relics, on the other hand, show the structures of synchrotron polarization vectors, organized over the scales of ∼ Mpc, challenging the turbulence origin of cluster magnetic fields. Unlike turbulence in the interstellar medium, turbulence in the ICM is subsonic. And it is driven sporadically in highly stratified backgrounds, when major mergers occur during the hierarchical formation of clusters. To investigate quantitatively the characteristics of turbulence dynamo in such ICM environment, we performed a set of turbulence simulations using a high-order-accurate, magnetohydrodynamic (MHD) code. We find that turbulence dynamo could generate the cluster magnetic fields up to the observed level from the primordial seed fields of 10 −15 G or so within the age of the universe, if the MHD description of the ICM could be extended down to ∼ kpc scales. However, highly organized structures of polarization vectors, such as those observed in the Sausage relic, are difficult to be reproduced by the shock compression of turbulence-generated magnetic fields. This implies that the modeling of giant radio relics may require the pre-existing magnetic fields organized over ∼ Mpc scales.

Time-resolved turbulent dynamo in a laser plasma

“…Possible explanations for why the characteristic scale of the magnetic fields in our experiment is larger than anticipated from resistive MHD simulations include additional physical processes which could arise due to the order-unity Hall parameter being attained subsequent to the seed field's amplification, or differences in the mechanism of resistive dissipation between the experiments and the simulations. The result is tantalizing given the long-standing problem of explaining the observed scale of tangled magnetic fields present in the ICM (57): current ICM simulations tend to predict magnetic fields at smaller scales than observed (58,59).…”

Essentially non-oscillatory and weighted essentially non-oscillatory schemes

“…The merger of compact stars in the twofamilies scenario was studied in Pietri et al (2019). The turbulence dynamo in the stratified medium of Galaxy clusters was simulated in Roh et al (2019).…”

Deep low-frequency radio observations of Abell 2256 I: The filamentary radio relic

“…It is challenging to understand the nature of filamentary structures in relics since the geometry/topology of the relativistic plasma is affected by projection effects. The observed filaments could be substructures of a com- plex shock front, possibly highlighting the underlying distribution of Mach numbers (Hong et al 2015;Roh et al 2019;Wittor et al 2019;Dominguez-Fernandez et al 2020) and a range of electron acceleration efficiencies by shock acceleration (Wittor et al 2017). Conversely, they can reflect a pattern of fluctuations in the magnetic field strength and topology at the corrugated surface of a shock.…”

“…A magenta dashed curve outlines the northwest edge of the relic emission detected in radio. The shock front identified by Ge et al (2020) via X-ray observations is marked with yellow dashed curve.shock surfaces of different Mach numbers: radio observations tend to pick the strongest Mach number shocks, while X-ray observations to that of low Mach number shocks along a given line of sight(Skillman et al 2013;Hong et al 2015;Roh et al 2019;Wittor et al 2019;Dominguez-Fernandez et al 2020). Recently,Wittor et al (2021) studied the radio versus X-ray Mach number discrepancy by comparing numerical simulations with the integrated radio spectra of the observed relics.…”