On the Small-scale Turbulent Dynamo in the Intracluster Medium: A Comparison to Dynamo Theory*

Abstract

We present non-radiative, cosmological zoom-in simulations of galaxy-cluster formation with magnetic fields and (anisotropic) thermal conduction of one massive galaxy cluster with M vir ∼ 2 × 1015 M ⊙ at z ∼ 0. We run the cluster on three resolution levels (1×, 10×, 25×), starting with an effective mass resolution of 2 × 108 M ⊙, subsequently increasing the particle number to reach 4 × 106 M ⊙. The maximum spatial resolution obtained in the simulations is limited by the gravitational softening reaching ϵ = 1.0 kpc at the highest resolution level, allowing one to resolve the hierarchical assembly of the structures in fine detail. All simulations presented are carried out with the SPMHD code gadget3 with an updated SPMHD prescription. The primary focus of this paper is to investigate magnetic field amplification in the intracluster medium. We show that the main amplification mechanism is the small-scale turbulent dynamo in the limit of reconnection diffusion. In our two highest resolution models we start to resolve the magnetic field amplification driven by the dynamo and we explicitly quantify this with the magnetic power spectra and the curvature of the magnetic field lines, consistent with dynamo theory. Furthermore, we investigate the ∇ · B = 0 constraint within our simulations and show that we achieve comparable results to state-of-the-art AMR or moving-mesh techniques, used in codes such as enzo and arepo. Our results show for the first time in a cosmological simulation of a galaxy cluster that dynamo action can be resolved with modern numerical Lagrangian magnetohydrodynamic methods, a study that is currently missing in the literature.

Inflationary and Phase-transitional Primordial Magnetic Fields in Galaxy Clusters

“…All of our models exhibit a degree of small-scale dynamo amplification, as hinted at by the lognormality of the magnetic field PDFs and the folded structures of the field lines (i.e., the anticorrelation between the field strength and curvature and the ordering of the characteristic wavenumbers). Consistent with previous works (Vazza et al 2018;Domínguez-Fernández et al 2019;Steinwandel et al 2022), we find that cosmological MHD simulations do not exhibit a smallscale dynamo that can be compared one-to-one to the Kazantsev theory. 4.…”

“…A comprehensive criterion for dynamo action in the presence of gravity is still missing; see Brandenburg & Ntormousi (2022) for some attempts. 12 We follow the diagnostics presented in Schekochihin et al (2004), which have also been used in Vazza et al (2018) and Steinwandel et al (2022).…”

A Formation Mechanism for “Wrong Way” Radio Relics

“…The parent simulation used a WMAP7 cosmology with Ω 0 = 0.24, Ω Λ = 0.76, Ω baryon = 0.04, h = 0.72, and σ 8 = 0.8, which we also adopt for the present simulation. We start the simulation at redshift z = 310 and seed a constant magnetic field in the x-direction with B 0 = 10 −14 G (see Steinwandel et al 2022 for a study of the impact of the choice of B 0 ). The initial conditions of this cluster at this resolution have been used to study the interaction between internal and accretion shocks in Zhang et al (2020aZhang et al ( , 2020b and its magnetic field has been studied in Steinwandel et al (2023).…”

“…In this work we propose that a possible formation mechanism for these "wrong way" relics (as they are referred to in Riseley et al 2022) is the collision of an outward traveling shock front with an infalling substructure. We investigate this scenario in the sibling simulation of an ultra-high-resolution MHD simulation of an M vir ≈ 1.3 × 10 15 M e galaxy cluster introduced in Steinwandel et al (2023), where we attached a population of CR protons and electrons to every resolution element of our simulation. This effectively turns every particle into a tracer particle for CRs, while also accounting for feedback by the CR component on the thermal gas.…”

Turbulent Magnetic Field Amplification by the Interaction of a Shock Wave and Inhomogeneous Medium

“…It takes into account the magnetic feedback on turbulence and can be generally applied to studying magnetic field amplification in diverse astrophysical processes. Xu & Lazarian (2016)ʼs theory explains the inefficient dynamo growth seen in simulations (Cho et al 2009) and has been previously tested with numerical simulations of magnetic field amplification during the formation of galaxy clusters and the first stars (Steinwandel et al 2021;Stacy et al 2022).…”