A Supernova-regulated Interstellar Medium: Simulations of the Turbulent Multiphase Medium

The dynamic state of the interstellar medium, heated and stirred by supernovae (SNe), is simulated using a three-dimensional, nonideal MHD model in a domain extended kpc horizontally and 2 kpc vertically, 0.5 # 0.5 with the gravitational field symmetric about the midplane of the domain, . We include both Type I and z 5 0 Type II SNe, allowing the latter to cluster in regions with enhanced gas density. The system segregates into two main phases: a warm, denser phase and a hot, dilute gas in global pressure equilibrium; there is also dense, cool gas compressed into filaments, shells, and clumps by expanding SN remnants. The filling factor of the hot phase grows with height, so it dominates at kpc. The multicomponent structure persists throughout the FzF * 0.5 simulation, and its statistical parameters show little time variation. The warm gas is in hydrostatic equilibrium, which is supported by thermal and turbulent pressures. The multiphase gas is in a state of developed turbulence. The rms random velocity is different in the warm and hot phases, 10 and 40 km s 21 , respectively, at FzF & 1 kpc; the turbulent cell size (twice the velocity correlation scale) is about 60 pc in the warm phase. Subject headings: galaxies: ISM — ISM: kinematics and dynamics — MHD — turbulence T. 10 phases, although molecular clouds and some transient phases can also be important in many respects. The main sources of energy maintaining this complex structure are supernova (SN) explosions and stellar winds. The energy ejected by the SNe not only supports the hot phase but also drives ubiquitous tur- bulence in all diffuse phases. Thus, turbulence and multiphase structure are intrinsically connected features of the ISM, and in this Letter, we present a model describing them in a self- consistent manner. 2. THE MODEL We model the ISM in the solar neighborhood using a local three-dimensional, nonideal MHD model, which includes the effects of density stratification in the Galactic gravity field, heating via supernova explosions, radiative cooling, large-scale shear due to Galactic differential rotation, compressibility, and magnetic fields, together with thermal conductivity and kinetic and magnetic viscosities. We adopt a local Cartesian frame of reference that rotates at an angular velocity of km s 21 Q 5 25 0 kpc 21 and assume a flat rotation curve. We solve for deviations u from this basic flow (Brandenburg et al. 1995). We solve the standard nonideal MHD equations, namely, the induction equa- tion written for the magnetic vector potential, the momentum equation, the energy equation, and the continuity equation. The vertical distribution of the gravitational acceleration is taken 1