Simulations of Magnetohydrodynamics Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction

Abstract

We perform a suite of simulations of cooling cores in clusters of galaxies in order to investigate the effect of the recently discovered heat flux buoyancy instability (HBI) on the evolution of cores. Our models follow the 3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture the effects of anisotropic heat conduction along the lines of magnetic field, but do not account for the cosmological setting of clusters or the presence of AGN. Our model clusters can be divided into three groups according to their final thermodynamical state: catastrophically collapsing cores, isothermal cores, and an intermediate group whose final state is determined by the initial configuration of magnetic field. Modeled cores that are reminiscent of real cluster cores show evolution towards thermal collapse on a time scale which is prolonged by a factor of ∼ 2 − 10 compared with the zeroconduction cases. The principal effect of the HBI is to re-orient field lines to be perpendicular to the temperature gradient. Once the field has been wrapped up onto spherical surfaces surrounding the core, the core is insulated from further conductive heating (with the effective thermal conduction suppressed to less than 10 −2 of the Spitzer value) and proceeds to collapse. We speculate that, in real clusters, the central AGN and possibly mergers play the role of "stirrers," periodically disrupting the azimuthal field structure and allowing thermal conduction to sporadically heat the core.

Interplay Among Cooling, Agn Feedback, and Anisotropic Conduction in the Cool Cores of Galaxy Clusters

“…Note that for these runs, without AGN feedback, anisotropic conduction alone is not enough to counteract radiative cooling, and thus cooling catastrophe occurs at t  0.3 Gyr. Also, previous simulations found that catastrophic cooling can be delayed when thermal conduction is included (e.g., Bogdanović et al 2009). We do not find a significant delay because the adopted profiles for the Perseus cluster havea short cooling time and flat temperature profile near the center.…”

“…This implies that little turbulence is generated by the HBI itself (Bogdanović et al 2009;McCourt et al 2011) or by the decay of g-modes.…”

“…Earlier simulations that investigated the effects of HBI (Bogdanović et al 2009;Parrish et al 2009;Ruszkowski & Oh 2010;McCourt et al 2011;Kunz et al 2012;Avara et al 2013) have focused on scenarios where the magnetic field is negligible (β > 10 5 ). We use a realistic magnetic field strength (β ∼ 100) for typical ICM and show that magnetic tension could suppress a significant portion of the HBI-unstable modes below a critical length scale (Equation (12)).…”

“…This may, in turn, promote the driving of turbulence in and subsequent heating of the ICM (this possibility was speculated on in Reynolds et al 2015). Somewhat less intuitively, anisotropic thermal conduction renders the CC unstable to the heat-flux-driven buoyancy instability (HBI ;Quataert 2008;Parrish et al 2009;McCourt et al 2011), which, in quiescent atmospheres with extremely weak fields, saturates by reorienting the field lines perpendicular to the temperature gradients, wrapping them onto the spherical isotherms (Bogdanović et al 2009;Parrish et al 2009). Thus, it might seem that even powerful thermal conduction is self-limiting, although semilocal simulations by Avara et al (2013) and Kunz et al (2012) have shown that the saturation property of the HBI is altered in more realistic magnetic field strengths, leading to radial magnetic filaments that can be long-lived conduits for heat flux.…”

“…One can readily imagine that the buoyantly rising jetblown cavities may stretch field lines in the radial direction, counteracting the field-line wrapping of the HBI and allowing conductive heat fluxes to flow. In the extreme, as speculated by Bogdanović et al (2009), one could imagine CCs in which the role of kinetic feedback was purely to stir the ICM and maintain open magnetic channels for the conductive heat flux. At the same time, thermal conduction may strongly suppress local thermal instabilities that are important for the condensation of cold gas that, ultimately, becomes the fuel for the AGN exerting the kinetic feedback.…”