Oral Presentation
Physical properties of the multiphase interstellar medium in converging HI flow
Presenter: Masato Kobayashi (Osaka University)
Constructing proper models for unresolved/sub-grid physics in galactic-scale simulations is important to understand time-evolution of the multiphase interstellar medium and subsequent star formation consistently along with galaxy evolution. To derive such description, especially focusing on the phase transition dynamics from warm to cold neutral medium on sub-pc scales, we perform a series of hydrodynamics simulations of colliding warm neutral medium flow with cooling and heating.
We observe the time evolution of the created shock-compressed layer while the flows continue more than 10 Myr. The layer initially expands almost adiabatically during one cooling-time (the first 1 Myr) and slows down and expands almost isothermally due to cooling. During this isothermal phase, cold neutral medium occupies about 60 percent of the mass in the layer whereas warm neutral medium occupies more than 50 percent of the volume. This mass fraction vary in time and reaches at warm:cold = 2:8 after 10Myr, which does not depend on the flow velocity. We derive an effective polytropic index of the entire shock-compressed layer, with which one can reproduce the same compressibility in pure warm neutral medium. This derivation essentially utilizes one-fluid Rankine-Hugoniot relation but with the density jump between post-/pre-shock regions and the layer expansion rate observed in our simulations.
The time-dependent mass fraction and effective index that we derive are key steps with which we are able to investigate time-evolution of the interstellar medium and star formation across different environments within/between galaxies.
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