On the small scale turbulent dynamo in the intracluster medium: A comparison to dynamo theory

Abstract

We present non-radiative, cosmological zoom-simulations of galaxy cluster formation with magnetic fields and (anisotropic) thermal conduction of one very massive galaxy cluster with a mass at redshift zero that corresponds to M vir ∼ 2 × 10 15 M . We run the cluster on three resolution levels (1X, 10X, 25X), starting with an effective mass resolution of 2 × 10 8 M , subsequently increasing the particle number to reach 4 × 10 6 M . The maximum spatial resolution obtained in the simulations is limited by the gravitational softening reaching = 1.0 kpc at the highest resolution level, allowing to resolve the hierarchical assembly of the structures in very fine detail. All simulations presented, have been carried out with the SPMHD-code gadget-3 with a heavily updated SPMHD prescription. The primary focus is to investigate magnetic field amplification in the Intracluster Medium (ICM). 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 this process and we explicitly quantify this with the magnetic power-spectra and the magnetic tension that limits the bending 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 fully cosmological simulation of a galaxy cluster that dynamo action can be resolved in the framework of a modern Lagrangian magnetohydrodynamic (MHD) method, a study that is currently missing in the literature.

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

“…However, on galaxy cluster scales in the ICM, turbulence driven by the structure formation processes and merger shocks will quickly generate a saturated magnetic field with a field strength of around ∼ µG on the scales of Mpc without the need for an explicit seeding of these fields by galactic winds (e.g. Vazza et al 2018;Steinwandel et al 2021). In turn, these fields will then be ordered on larger scales by the structure formation process itself with some evidence that the Void magnetic field can "remember" some of the strucutre of the initial seed field on the scales of a few 10 Mpc (e.g.…”

Evolution of Primordial Magnetic Fields during Large-scale Structure Formation

“…The spatial resolution adopted in the present simulations place limitations on our results. Our resolution is not sufficient to resolve, for example, the additional magnetic amplification within galaxy clusters (Xu et al 2009;Vazza et al 2018;Steinwandel et al 2021). Nevertheless, in Appendix B we show that our results are robust at least on scales  1 Mpc h -1 .…”

“…Regardless of their origin, the physical mechanisms that further amplify magnetic fields during cosmological structure formation are adiabatic contraction and turbulent dynamo. The latter is the preferred mechanism for explaining the efficient amplification of magnetic fields on galaxy (see, e.g., Pakmor et al 2017;Rieder & Teyssier 2017;Steinwandel et al 2020) and galaxy-cluster scales (see, e.g., Xu et al 2009;Vazza et al 2018;Domínguez-Fernández et al 2019;Steinwandel et al 2021). Nevertheless, distinguishing between different magnetogenesis scenarios in the high-density regions of the cosmic web could be complicated because the memory of the seed field is believed to be erased by the turbulent dynamo (Cho 2014).…”

Magnetogenesis and the Cosmic Web: A Joint Challenge for Radio Observations and Numerical Simulations

“…In typical ICM conditions (density n ≥ 10 −4 [cm −3 ], temperature T ∼ 10 7 -10 8 K and plasma beta β pl ∼ 10 2 ), a dynamo can bring the magnetic energy density close to equipartition with the turbulent plasma kinetic energy in galaxy clusters. A small-scale dynamo also erases the memory of the initial magnetic field level, as verified by direct numerical simulations e.g., [9][10][11][12][13][14][15][16][17]. While all previous results are based on ideal magneto-hydrodynamical (MHD) simulations, kinetic simulations have recently started to explore the more realistic situation of weakly collisional plasmas.…”