Cosmic magnetic fields with masclet: an application to galaxy clusters

Abstract

We describe and test a new version of the adaptive mesh refinement (AMR) cosmological code MASCLET. The new version of the code includes all the ingredients of its previous version plus a description of the evolution of the magnetic field under the approximation of the ideal magneto-hydrodynamics (MHD). To preserve the divergence-free condition of MHD, the original divergence cleaning algorithm of Dedner et al. (2002) is implemented. We present a set of well-known 1D and 2D tests, such as several shock-tube problems, the fast rotor and the Orszag-Tang vortex. The performance of the code in all the tests is excellent with estimated median relative errors of ∇ · B in the 2D tests smaller than 5 × 10 −5 for the fast rotor test, and 5 × 10 −3 for the Orszag-Tang vortex. As an astrophysical application of the code, we present a simulation of a cosmological box of 40 comoving Mpc side length in which a primordial uniform comoving magnetic field of strength 0.1 nG is seeded. The simulation shows how the magnetic field is channelled along the filaments of gas and is concentrated and amplified within galaxy clusters. Comparison with the values expected from pure compression reveals an additional amplification of the magnetic field caused by turbulence in the central region of the cluster. Values of the order of ∼ 1µG are obtained in clusters at z ∼ 0 with median relative errors of ∇ · B below 0.4%. The implications of a proper description of the dynamics of the magnetic field and their possible observational counterparts in future facilities are discussed.

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.…”

“…Although the use of adaptive mesh refinement is crucial to resolve the turbulent flow inside forming structures and its related dynamo amplification e.g., [12,16], for the goal of this investigation, we only used a uniform grid approach on large volumes, as they are overall more suitable to model strong accretion shocks in low density regions, and to follow the topology of magnetic fields in voids e.g., [103].…”

“…However, this simplistic method reproduced the results obtained by other methods e.g., [7,65,130]. In these runs, we adopt the same amplification efficiency calibrated in [65] for the DYN5 run, but in all other new runs (P, CMB2, CMB3, CMB4), we allowed the sub-grid model to be activated only for ρ ≥ 50 ρ , i.e., only within the virial radius of halos, where turbulence is predicted to be well developed and mostly solenoidal, and we increased by a factor 10 the amplification for z ≤ 1, where the virialisation of halos is also expected to maximise the amplification of magnetic fields via small-scale dynamo e.g., [16,151].…”

Cosmological magnetogenesis: the Biermann battery during the Epoch of reionization

“…We see that the 𝑛 2/3 scaling is not strictly reproduced by our simulation results. This is usually attributed to turbulent processes providing additional magnetic field amplification, on top of bare gravitational contraction (Dubois & Teyssier 2008;Quilis et al 2020). This relatively weak effect can be interpreted as the very first sign of a galactic dynamo (see Discussion below).…”

Magnetogenesis and the Cosmic Web: a joint challenge for radio observations and numerical simulations

“…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.…”