Suppressed heat conductivity in the intracluster medium: implications
  for the magneto-thermal instability
  In the outskirts of the intracluster medium (ICM) in galaxy clusters, the
temperature decreases with radius. Due to the weakly collisional nature of the
plasma, these regions are susceptible to the magneto-thermal instability (MTI),
which can sustain turbulence and provide turbulent pressure support in the ICM.
This instability arises due to heat conduction directed along the magnetic
field, with a heat conductivity which is normally assumed to be given by the
Spitzer value. Recent numerical studies of the ion mirror and the electron
whistler instability using particle-in-cell codes have shown that microscale
instabilities can lead to a reduced value for the heat conductivity in the ICM.
This could in turn influence the efficiency with which the MTI drives
turbulence. In this paper we investigate the influence of reduced heat
transport on the nonlinear evolution of the MTI. We study plane-parallel,
initially static atmospheres and employ a subgrid model that mimics the
influence of the mirror instability on the heat conductivity. We use this
subgrid model to assess the effect of microscales on the large scale dynamics
of the ICM. We find that the nonlinear saturation of the MTI is surprisingly
robust in our simulations. Over a factor of $\sim 10^3$ in the
thermal-to-magnetic pressure ratio and collisionality we find at most modest
changes to the saturation of the MTI with respect to reference simulations
where heat transport is unsuppressed.
