MAGNETIC AND THERMAL PRESSURES IN TURBULENT TWO-DIMENSIONAL BISTABLE FLOWS

Abstract

We present results from a two-dimensional numerical study of the effects of turbulent velocity fluctuations on the thermal and magnetic pressure distribution of a magnetized thermally bistable flow in a 100 × 100 pc2 box. We analyze simulations with a uniform initial magnetic field whose intensity, B 0, varies from 2.5 to 10 μG and in which the turbulent forcing is driven at scales ranging from 6.25 to 50 pc and with Mach numbers, with respect to the gas at 104 K M, ranging from 0.5 to 3.0. We find that the response of the thermal pressure distribution to variations on the forcing scale and Mach number follows the same trends as in the nonmagnetic case, which is consistent with a reduction of the ratio between the turbulent crossing time and the cooling time produced when the Mach number is increased or when the forcing scale is decreased. In the magnetic case, however, the thermal pressure distribution develops a low-pressure tail and does not develop a power-law shape. We interpret this fact as a consequence of the magnetic pressure acting as a random force instead of as a restoring force. On the other hand, the magnetic pressure distribution is very sensitive to the Mach number which in fact determines the maximum value of the magnetic pressure independently of B 0. For low-density gas, less than 80 cm–3, we do not observe any correlation between the density and the magnetic field strength but the densest gas in our simulations shows magnetic pressures equal to or above the pressure at B 0 and thermal pressures equal to or above the value given by the mean density in thermal equilibrium. The high thermal pressure of this gas suggests that it has a dynamical origin.