Abstract
ABSTRACT Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question by using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disc galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyse how gravity determines the vertical pressure profile as well as how the total ISM pressure is partitioned between different phases and components (thermal, dispersion/turbulence, and bulk flows). We show that, on average and consistent with previous more idealized simulations, the total ISM pressure balances the weight of the overlying gas. Deviations from vertical pressure balance increase with increasing galactocentric radius and with decreasing averaging scale. The different phases are in rough total pressure equilibrium with one another, but with large deviations from thermal pressure equilibrium owing to kinetic support in the cold and warm phases, which dominate the total pressure near the mid-plane. Bulk flows (e.g. inflows and fountains) are important at a few disc scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total mid-plane pressure is well-predicted by the weight of the disc gas and we show that it also scales linearly with the star formation rate surface density (ΣSFR). These results support the notion that the Kennicutt–Schmidt relation arises because ΣSFR and the gas surface density (Σg) are connected via the ISM mid-plane pressure.
Highlights
1.1 ISM pressure balance and its connection to the regulation of star formation in galaxiesUnderstanding the structure and dynamics of the interstellar medium (ISM) has long been recognized as a fundamental problem in astrophysics
We define a framework within which we measure the physical properties of different ISM phases, with the goal of testing whether vertical pressure balance is realized in different regions of the galaxy
In particular, the degree to which the ISM pressure can be modelled as being in quasi-hydrostatic balance with the weight of the overlying gas, an assumption which is the basis of many models for the structure of disc galaxies (e.g. Boulares & Cox 1990) as well as models of the origin of the KS relation (e.g. Thompson et al 2005; Ostriker & Shetty 2011; Faucher-Giguere et al 2013; Hayward & Hopkins 2017; Orr et al 2018)
Summary
1.1 ISM pressure balance and its connection to the regulation of star formation in galaxies. Since the ISM is the reservoir out of which stars form, the phase structure, dynamics, and thermodynamics of interstellar gas directly affect the process of star formation (e.g. McKee & Ostriker 2007; Krumholz 2014). Beyond the structure of the ISM, our interest in pressure balance is motivated by the role this principle may play in regulating star formation on galactic scales. Many galaxies drive powerful outflows that eject gas from the ISM into the IGM (Steidel et al 2010; Muratov et al 2015) These complexities are typically neglected in existing analytic models (see Hayward & Hopkins 2017, for an exception), but their effects can be investigated using simulations which include some or all of these processes. Presented is a brief analysis of the validity of equilibrium models for explaining the KS relation that emerge in such simulations; this will be expanded and built upon in future papers
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