Thursday, July 18, 2013

1307.4397 (M. Gaspari et al.)

Constraining turbulence and conduction in the hot ICM through density perturbations    [PDF]

M. Gaspari, E. Churazov
Using 3D high-resolution hydrodynamic simulations, we study the interplay of varying conduction and turbulence in the hot intracluster medium, tracking both electrons and ions. We exploit the power spectrum of the relative gas density perturbations, to precisely constrain conduction and turbulence. The normalization of the characteristic amplitude of perturbations determines the strength of turbulence, since it is linearly related to the Mach number: A_max = 0.25 M. The slope of the spectrum defines the level of conduction. In a non-conductive medium, subsonic stirring motions generate a density `cascade' which is nearly Kolmogorov. Instead, increasing conduction (with magnetic suppression f = 0.001 -> 1) progressively steepens the spectrum towards the sharp Burgers regime. The turbulent Prandtl number defines the dynamic similarity of the flow. The threshold P_t < 100 indicates where the spectrum has a significant decay. The transition is gentle for strong suppression of conduction, f < 0.001, while sharp in the opposite regime. For strong conductivity (f > 0.1), P_t ~ 100 occurs on spatial scales larger than the injection scale. This regime would be also manifest in the SB_x or residual images, in which K-H and R-T rolls and filaments are washed out, preserving the smooth shape of the cluster. In a stratified system, realistic perturbations show a mixture of modes: weak turbulence induces more isobaric fluctuations, strong turbulence enhances the adiabatic mode, while conduction forces the isothermal regime. We provide a general model, which is applied to very deep observations of Coma. The observed spectrum indicates a strongly suppressed effective conduction, f = 0.001, and mild subsonic turbulence, M = 0.45. The low conductivity corroborates the survival of sharp features in the ICM (cold fronts, filaments, bubbles), and implies that cooling flows can not be balanced by conduction.
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