Understanding gas mixing in the circumgalactic medium

Abstract

We study gas mixing in a simulated Milky Way-mass galaxy's circumgalactic medium (CGM) using cosmological `zoom-in' simulations. We insert tracer dyes in the CGM with different gas flows (shearing, coherent, and static) and diverse physical properties to track gas mixing. We correlate the extent and shape of the dye spread with the local gas properties to understand gas mixing. Velocity dispersion and traceless symmetric shear tensors (pure shear deformation) in small regions (<= 5 kpc) around the dye injection locations best predict the dye spread extent after 200 Myr. We use this to determine diffusion calibration constants for subgrid-scale mixing models. While the dye shape after 200 Myr aligns well with the velocity dispersion and magnetic field dispersion, the best alignment occurs with the dispersion of stretching eigenvectors (traceless symmetric shear tensor) and plane-of-rotation (antisymmetric shear or vorticity tensor) in large regions (10 kpc) around the dye injection locations. Therefore, shear statistics and velocity dispersion best predict the extent and shape of mixed gas. The linear temporal dependence of the dye spread suggests superdiffusion in the CGM, potentially due to turbulent and large-scale coherent flows or numerical diffusion. Despite significant numerical mixing from our 1 kpc resolution (insufficient to resolve Reynolds numbers ~10^2-10^3, which require a few hundred pc resolution), our correlation results are robust thanks to fixed spatial resolution throughout the CGM. These results can be used to predict diffusion coefficients to model magnetic field diffusion, heat transport, and metal mixing.