Cooling flows around cold clouds in the circumgalactic medium: steady-state models & comparison with TNG50

Abstract

Cold, non-self-gravitating clumps occur in various astrophysical systems, ranging from the interstellar and circumgalactic medium (CGM), to AGN outflows and solar coronal loops. Cold gas has diverse origins such as turbulent mixing or precipitation from hotter phases. We obtain the analytic solution for a steady pressure-driven 1-D cooling flow around cold, local over-densities, irrespective of their origin. Our solutions describe the slow and steady radiative cooling-driven gas inflow in the saturated regime of nonlinear thermal instability in clouds, sheets and filaments. Such a cooling flow develops when the gas around small clumps undergoes radiative cooling. These small-scale, cold `seeds' are embedded in a large volume-filling hot CGM maintained by feedback. We use a simple two-fluid treatment to include magnetic fields as an additional polytropic fluid. To test the limits of applicability of these analytic solutions, we compare with the gas structure found in and around small-scale cold clouds in the CGM of massive halos in the TNG50 cosmological MHD simulation from the IllustrisTNG suite. Despite qualitative resemblance of the gas structure, we find deviations from steady state profiles generated by our model. Complex geometries and turbulence all add complexity beyond our analytic solutions. We derive an exact relation between the mass cooling rate (${\dot{\mathrm{M}}}_{\mathrm{cool}}$) and the radiative cooling rate (${\dot{\mathrm{E}}}_{\mathrm{cool}}$) for a steady cooling flow. A comparison with the TNG50 clouds shows that this cooling flow relation only applies in a narrow temperature range around $\sim 10^{4.5}$ K where the isobaric cooling time is the shortest. In general, turbulence and mixing, instead of radiative cooling, may dominate the transition of gas between different temperature phases.

arXiv comment11 pages, 10 figures; comments and suggestions are welcome; accepted for publication in MNRAS; Github repo: https://github.com/dutta-alankar/cooling-flow-model.git YouTube: https://www.youtube.com/channel/UCPw9R4ReQqAHq8ArgJhFWLA

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• The Circumgalactic Medium of Milky Way-like Galaxies in the TNG50 Simulation -- II: Cold, Dense Gas Clouds and High-Velocity Cloud Analogs
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• GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption
• Characterising the turbulent multiphase halos with periodic box simulations
• Galactic coronae in Milky Way-like galaxies: the role of stellar feedback in gas accretion
• High-Spectral Resolution Observations of the Optical Filamentary Nebula in NGC 1275
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• Fading in the Flow: Suppression of cold gas growth in expanding galactic outflows
• The Thermodynamic and Kinematic Evolution of Circumgalactic Gas around $z=1$ in the IllustrisTNG model