Cosmological Magnetohydrodynamic Simulations of Cluster Formation With Anisotropic Thermal Conduction

Abstract

The intracluster medium (ICM) has been suggested to be buoyantly unstable in the presence of magnetic field and anisotropic thermal conduction. We perform first cosmological simulations of galaxy cluster formation that simultaneously include magnetic fields, radiative cooling, and anisotropic thermal conduction. In isolated and idealized cluster models, the magnetothermal instability (MTI) tends to reorient the magnetic fields radially whenever the temperature gradient points in the direction opposite to gravitational acceleration. Using cosmological simulations of cluster formation we detect radial bias in the velocity and magnetic fields. Such radial bias is consistent with either the inhomogeneous radial gas flows due to substructures or residual MTI-driven field rearrangements that are expected even in the presence of turbulence. Although disentangling the two scenarios is challenging, we do not detect excess bias in the runs that include anisotropic thermal conduction. The anisotropy effect is potentially detectable via radio polarization measurements with LOFAR and the Square Kilometer Array and future X-ray spectroscopic studies with the International X-ray Observatory. We demonstrate that radiative cooling boosts the amplification of the magnetic field by about two orders of magnitude beyond what is expected in the non-radiative cases. This effect is caused by the compression of the gas and frozen-in magnetic field as it accumulates in the cluster center. At z = 0 the field is amplified by a factor of about 10 6 compared to the uniform magnetic field that evolved due to the universal expansion alone. Interestingly, the runs that include both radiative cooling and thermal conduction exhibit stronger magnetic field amplification than purely radiative runs. In these cases, buoyant restoring forces depend on the temperature gradients rather than the steeper entropy gradients. Thus, the ICM is more easily mixed and the winding up of the frozen-in magnetic field is more efficient, resulting in stronger magnetic field amplification. We also demonstrate that thermal conduction partially reduces the gas accretion driven by overcooling despite the fact that the effective conductivity is suppressed below the Spitzer-Braginskii value.

Increasing Black Hole Feedback-induced Quenching with Anisotropic Thermal Conduction

“…The initial conditions for this cluster were generated from the Millennium XXL simulation (Angulo et al 2012) and then rescaled to the latest Wilkinson Microwave Anisotropy Probe-9 (WMAP-9) measurements (Hinshaw et al 2013 , respectively, with a softening length of 1.4 kpc for both particle types. Our mass resolution is ∼1000 times and our spatial resolution is ∼30 times better than those of previous simulations attempting to model anisotropic thermal conduction in a cosmological context (Ruszkowski et al 2011). We also achieve better resolution than idealized non-cosmological simulations with thermal conduction (Ruszkowski & Oh 2010;Parrish et al 2012;Yang & Reynolds 2016) and recent cosmological pure hydrodynamic simulations of clusters (Hahn et al 2017;Rasia et al 2015).…”

“…Recent simulations (Ruszkowski et al 2011;Yang & Reynolds 2016) have shown that thermal conduction alone is not strong enough to offset the cooling losses, even if assuming a full Spitzer conduction coefficient along magnetic field lines. It may, however, provide part of the heating, reducing the burden on the black hole (Yang & Reynolds 2016).…”

Turbulence in the ICM from mergers, cool-core sloshing, and jets: results from a new multi-scale filtering approach

“…While its inclusion is not expected to change the dynamics of turbulent motions driven by large-scale mergers and accretion on kpc scales (e.g. Xu et al 2009;Ruszkowski et al 2011;Bonafede et al 2011), local amplification of B in shear flows can suppress the growth of instabilities and mixing motions along the spiral arms of sloshing structures (ZuHone et al 2011b) and along AGN-jets (O'Neill & Jones 2010).…”

Magnetic Fields in Diffuse Media

“…Xu et al 2009;Ruszkowski et al 2011;Bonafede et al 2011a), local amplification of the magnetic field in shear To resolve the turbulence excited by cluster mergers, sloshing and AGN in the same simulation, one would need to cover scales ranging from R vir 3 Mpc down to the presumed scale of physical dissipation at 0.1 kpc. While its inclusion is not expected to change the dynamics of turbulent motions driven by large-scale mergers and accretion on kpc scales (e.g.…”

“…Various authors have also estimated the amount of turbulence that is produced by the motions of the galaxies through the ICM (Faltenbacher et al 2005;Kim 2007;Ruszkowski et al 2011). In general, cluster galaxies can produce turbulence by two means: (1) As the cold, dense interstellar medium pushes against the external ICM, it produces vorticity, and (2) the gravity of the galaxy causes disturbances in the ICM, similar to dynamic friction.…”