Simulation movies
high-resolution 2d simulations of thermal instability in a stratified medium
2D density slices of thermal instability in a gravitationally stratified medium. In thermally unstable gas, small isobaric perturbations form cold, dense gas that precipitates onto the galaxy (simulation midplane). Cosmic ray pressure changes the morphology of cold gas that forms through thermal instability and reduces the inflow rate of cold gas. Simulation domain: 86 x 86 kpc, resolved by 4096 x 4096 cells. Initial cooling time / free-fall time = 0.3, and both movies are evolved for 17 cooling cycles. The simulation with cosmic rays was initialized so that cosmic ray pressure was equal to the gas pressure everywhere.
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Thermal instability
Thermal instability with cosmic rays
Feel free to use these wherever, but please cite them as Butsky et al. 2020
Thermal instability comparisons
The movies to the right are animations of some figures from Butsky et al., 2020. They model the formation and precipitation of cold gas through thermal instability in a gravitationally stratified medium, analogous to the CGM of an L* galaxy. The physical extent of each box is 43 kpc x 80 kpc and all movies are evolved for 10 cooling cycles,(which can correspond to different physical times for simulations with different ratios of the cooling time - to free fall time (tcool / tff)). All simulations are initialized with weak magnetic fields (plasma beta = 100). The direction of gravity is down.
The first movie (top) shows thermal instability without cosmic rays. The columns are organized by the efficiency of cooling in the simulations from highest (left) to lowest (right). When cooling is efficient, the simulation forms many cold gas cloudlets that precipitate. When cooling is inefficient, no cold gas forms.
The second and third panels show the evolution of the 2D density and temperature in thermally unstable gas for different initial cosmic ray pressures, increasing from left to right. All movies have tcool/tff = 0.3. With increased cosmic ray pressure, cold gas clouds become larger and less dense. This is because cosmic ray pressure counteracts compressive heating, allowing gas to cool without condensing as much. The temperature of the cold gas stays the same because the temperature of cold gas phase is set by the cooling curve, which is an approximation to atomic physics and is insensitive to cosmic ray pressure. When cosmic ray pressure is strong relative to gas pressure, it can prevent cold gas from precipitating onto the galaxy.
The last movie (bottom) shows the evolution of the 2D slice of cosmic ray pressure for simulations with tcool/tff = 0.3, and Pc/Pg = 1. This time, the columns vary by cosmic ray transport prescription: diffusion with two different diffusion coefficients and streaming at two different initial magnetic field strengths. Cosmic ray transport redistributes cosmic ray pressure from regions of high density to regions of low density. When cosmic ray transport is very efficient, it can decouple cosmic ray pressure from the gas. Simulations with cosmic ray transport tend to have cold gas properties that are in-between those of simulations without cosmic rays and simulations with cosmic rays but without cosmic ray transport.
Downloads:
Thermal instability density projection
Thermal instability+CR: density slice
Thermal instability+CR: temperature slice
Thermal instability+CR transport
Feel free to use these wherever, but please cite them as Butsky et al., 2020
