Radiative turbulent mixing layers at high Mach numbers

TL;DR

This study uses 3D hydrodynamic simulations to explore how radiative turbulent mixing layers behave at high Mach numbers, revealing a two-zone structure and saturation effects in observable properties.

Contribution

It provides new insights into the Mach number dependence of TML properties, especially at high Mach numbers, and distinguishes the structure and cooling mechanisms from low Mach number regimes.

Findings

High Mach number TMLs develop a two-zone structure with a Mach-independent mixing zone.

Surface brightness and ion column densities saturate at Mach numbers above 1.

Inflow velocities and hot gas entrainment are suppressed at high Mach numbers.

Abstract

Radiative turbulent mixing layers (TMLs) are ubiquitous in astrophysical environments, e.g., the circumgalactic medium (CGM), and are triggered by the shear velocity at interfaces between different gas phases. To understand the shear velocity dependence of TMLs, we perform a set of 3D hydrodynamic simulations with an emphasis on the TML properties at high Mach numbers $\mathcal{M}$. Since the shear velocity in mixing regions is limited by the local sound speed of mixed gas, high-Mach number TMLs develop into a two-zone structure: a Mach number-independent mixing zone traced by significant cooling and mixing, plus a turbulent zone with large velocity dispersions which expands with greater $\mathcal{M}$. Low-Mach number TMLs do not have distinguishable mixing and turbulent zones. The radiative cooling of TMLs at low and high Mach numbers is predominantly balanced by enthalpy consumption and turbulent dissipation respectively. Both the TML surface brightness and column densities of intermediate-temperature ions (e.g., O VI) scale as $\propto\mathcal{M}^{0.5}$ at $\mathcal{M} \lesssim 1$, but reach saturation ($\propto \mathcal{M}^0$) at $\mathcal{M} \gtrsim 1$. Inflow velocities and hot gas entrainment into TMLs are substantially suppressed at high Mach numbers, and strong turbulent dissipation drives the evaporation of cold gas. This is in contrast to low-Mach number TMLs where the inflow velocities and hot gas entrainment are enhanced with greater $\mathcal{M}$, and cold gas mass increases due to the condensation of entrained hot gas.