Three-phase

Alex Hill : Why Is There a Three-Phase ISM?

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• published: 23 Sep 2016
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Conference: Star Formation, Magnetic Fields, and Diffuse Matter in the Galaxy: A Conference Honoring the Contributions of Richard Crutcher & Carl Heiles, May 23-26, 2016, Madison, WI, USA Website: http://www.astro.wisc.edu/ch16/ Conference Playlist: https://www.youtube.com/playlist?list=PLR0icYcBhA7Rv-QymeskXF15Kd-Mnh_T2 Date: Wednesday, May 25, 2016 Session: III. ISM/Simulations/Turbulence I (Chair: Blakesley Burkhart) Speaker: Alex Hill (Haverford College) Title: Why Is There a Three-Phase ISM? Abstract: I use insights from numerical simulations of supernova-driven turbulence with photoelectric heating and radiative cooling in a vertically-stratified interstellar medium to ask why the ISM is typically found to exist in three phases. Because the cold (~100 K, CNM) and warm (~10^4 K, WNM) phases can only coexist if the thermal pressure is in a narrow range, it is surprising that galaxies usually have both cold and warm gas in large quantities. In particular, the necessary thermal pressure is significantly lower than the typical hydrostatic pressure (with the balance made up by kinetic, magnetic, and cosmic ray pressures). So how does the thermal pressure "know" to be in the "right" stable two-phase range? We find that, for a wide variety of supernova rates, surface mass densities, and surface gravities, the thermal pressure of all three phases is confined to the narrow range of pressures in which cold and warm gas can coexist. This includes the ~10^6 K hot gas, which is not directly affected by the processes which set the two-phase equilibrium pressure. This is true even when the volume is more than 90% hot gas, in which case the warm and cold gas is confined to small, isolated pockets unless there is no vertical stratification to act as a pressure valve. I will discuss the physical mechanisms which keep the pressure in the WNM-CNM equilibrium regime.