Taming the TuRMoiL: The Temperature Dependence of Turbulence in Cloud–Wind Interactions

Abstract

Turbulent radiative mixing layers play an important role in many astrophysical contexts where cool (≲10 ^4 K) clouds interact with hot flows (e.g., galactic winds, high-velocity clouds, infalling satellites in halos and clusters). The fate of these clouds (as well as many of their observable properties) is dictated by the competition between turbulence and radiative cooling; however, turbulence in these multiphase flows remains poorly understood. We have investigated the emergent turbulence arising in the interaction between clouds and supersonic winds in hydrodynamic enzo-e simulations. In order to obtain robust results, we employed multiple metrics to characterize the turbulent velocity, v _turb . We find four primary results when cooling is sufficient for cloud survival. First, v _turb manifests clear temperature dependence. Initially, v _turb roughly matches the scaling of sound speed on temperature. In gas hotter than the temperature where cooling peaks, this dependence weakens with time until v _turb is constant. Second, the relative velocity between the cloud and wind initially drives rapid growth of v _turb . As it drops (from entrainment), v _turb starts to decay before it stabilizes at roughly half its maximum. At late times, cooling flows appear to support turbulence. Third, the magnitude of v _turb scales with the ratio between the hot phase sound-crossing time and the minimum cooling time. Finally, we find tentative evidence for a length scale associated with resolving turbulence. Underresolving this scale may cause violent shattering and affect the cloud’s large-scale morphological properties.

Keywords

• Galaxy evolution
• Hydrodynamical simulations
• Interstellar clouds
• Circumgalactic medium
• Galactic winds