Supernova-regulated ISM: the effects of radiative cooling and thermal conductivity on the multi-phase structure (original) (raw)

A Supernova Regulated Ism: 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 non-ideal MHD model in a domain extended 0:5 0:5 kpc horizontally and 2 kpc vertically, with gravitational eld symmetric about the midplane of the domain, z = 0. We include both Type I and 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 laments, shells and clumps by expanding SN remnants.

A supernova-regulated interstellar medium: Simulations of the turbulent multiphase medium

Astrophysical Journal, 1999

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 ϭ 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 տ 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 Ϫ1 , respectively, at FzF Շ 1 kpc; the turbulent cell size (twice the velocity correlation scale) is about 60 pc in the warm phase.

Turbulent structure of a stratified supernova-driven interstellar medium

The Astrophysical Journal, 2006

To study how supernova feedback structures the turbulent interstellar medium, we construct 3D models of ver-tically stratified gas stirred by discrete supernova explosions, including vertical gravitational fields and parameterized heating and cooling. The models ...

VERTICAL STRUCTURE OF A SUPERNOVA-DRIVEN TURBULENT, MAGNETIZED INTERSTELLAR MEDIUM

The Astrophysical Journal, 2012

Stellar feedback drives the circulation of matter from the disk to the halo of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a vertical column of the interstellar medium with initial conditions typical of the solar circle in which supernovae drive turbulence and determine the vertical stratification of the medium. The simulations were run using a stable, positivity-preserving scheme for ideal MHD implemented in the FLASH code. We find that the majority (≈ 90%) of the mass is contained in thermally-stable temperature regimes of cold molecular and atomic gas at T < 200 K or warm atomic and ionized gas at 5000 K < T < 10 4.2 K, with strong peaks in probability distribution functions of temperature in both the cold and warm regimes. The 200 − 10 4.2 K gas fills 50−60% of the volume near the plane, with hotter gas associated with supernova remnants (30−40%) and cold clouds (< 10%) embedded within. At |z| ∼ 1 − 2 kpc, transition-temperature (10 5 K) gas accounts for most of the mass and volume, while hot gas dominates at |z| > 3 kpc. The magnetic field in our models has no significant impact on the scale heights of gas in each temperature regime; the magnetic tension force is approximately equal to and opposite the magnetic pressure, so the addition of the field does not significantly affect the vertical support of the gas. The addition of a magnetic field does reduce the fraction of gas in the cold (< 200 K) regime with a corresponding increase in the fraction of warm (∼ 10 4 K) gas. However, our models lack rotational shear and thus have no largescale dynamo, which reduces the role of the field in the models compared to reality. The supernovae drive oscillations in the vertical distribution of halo gas, with the period of the oscillations ranging from ≈ 30 Myr in the T < 200 K gas to ∼ 100 Myr in the 10 6 K gas, in line with predictions by Walters & Cox.

MHD Simulations of a Supernova-driven ISM and the Warm Ionized Medium

2011

We present new 3D magnetohydrodynamic (MHD) simulations of a supernovadriven, stratified interstellar medium. These simulations were run using the Waagan et al. (2011) positivity preserving scheme for ideal MHD implemented in the Flash code. The scheme is stable even for the Mach numbers approaching 100 found in this problem. We have previously shown that the density distribution arising from hydrodynamical versions of these simulations creates low-density pathways through which Lyman continuum photons can travel to heights |z| > 1 kpc. This naturally produces the warm ionized medium through photoionization due primarily to O stars near the plane. However, our earlier models reproduce the peak but not the width of the observed emission measure distribution. Here, we examine whether inclusion of magnetic fields and a greater vertical extent to the simulation domain produce a gas distribution that better matches the observations. We further study the change of magnetic energy over time in our models, showing that it appears to reach a steady state after a few hundred megayears, presumably supported by a turbulent dynamo driven by the supernova explosions.

The Distribution of Pressures in a Supernova‐driven Interstellar Medium. I. Magnetized Medium

The Astrophysical Journal, 2005

Observations have suggested substantial departures from pressure equilibrium in the interstellar medium (ISM) in the plane of the Galaxy, even on scales under 50 pc. Nevertheless, multi-phase models of the ISM assume at least locally isobaric gas. The pressure then determines the density reached by gas cooling to stable thermal equilibrium. We use numerical models of the magnetized ISM to examine the consequences of supernova driving for interstellar pressures. In this paper we examine a (200 pc) 3 periodic domain threaded by magnetic fields. Individual parcels of gas at different pressures reach widely varying points on the thermal equilibrium curve: no unique set of phases is found, but rather a dynamically-determined continuum of densities and temperatures. A substantial fraction of the gas remains entirely out of thermal equilibrium. Our results appear consistent with observations of interstellar pressures. They also suggest that the high pressures observed in molecular clouds may be due to ram pressures in addition to gravitational forces. Much of the gas in our model lies far from equipartition between thermal and magnetic pressures, with ratios ranging from 0.1 to 10 4 and ratios of uniform to fluctuating magnetic field of 0.5-1. Our models show broad pressure probability distribution functions with log-normal functional forms produced by both shocks and rarefaction waves, rather than power-law distributions produced by isolated supernova remnants.

DEPENDENCE OF INTERSTELLAR TURBULENT PRESSURE ON SUPERNOVA RATE

The Astrophysical Journal, 2009

Feedback from massive stars is one of the least understood aspects of galaxy formation. We perform a suite of vertically stratified local interstellar medium (ISM) models in which supernova rates and vertical gas column densities are systematically varied based on the Schmidt-Kennicutt law. Our simulations have a sufficiently high spatial resolution (1.95 pc) to follow the hydrodynamic interactions among multiple supernovae that structure the interstellar medium. At a given supernova rate, we find that the mean mass-weighted sound speed and velocity dispersion decrease as the inverse square root of gas density. The sum of thermal and turbulent pressures is nearly constant in the midplane, so the effective equation of state is isobaric. In contrast, across our four models having supernova rates that range from one to 512 times the Galactic supernova rate, the mass-weighted velocity dispersion remains in the range 4-6 km s −1 . Hence, gas averaged over ∼100 pc regions follows P ∝ ρ α with α ≈ 1, indicating that the effective equation of state on this scale is close to isothermal. Simulated H I emission lines have widths of 10-18 km s −1 , comparable to observed values. In our highest supernova rate model, superbubble blow-outs occur, and the turbulent pressure on large scales is 4 times higher than the thermal pressure. We find a tight correlation between the thermal and turbulent pressures averaged over ∼100 pc regions in the midplane of each model, as well as across the four ISM models. We construct a subgrid model for turbulent pressure based on analytic arguments and explicitly calibrate it against our stratified ISM simulations. The subgrid model provides a simple yet physically motivated way to include supernova feedback in cosmological simulations.

Energy Input and Mass Redistribution by Supernovae in the Interstellar Medium

The Astrophysical Journal, 1998

We present the results of numerical studies of supernova remnant evolution and their effects on galactic and globular cluster evolution. We show that parameters such as the density and the metallicity of the environment significantly influence the evolution of the remnant, and thus change its effects on the global environment (e.g., globular clusters, galaxies) as a source of thermal and kinetic energy.

Turbulent structure and star formation in a stratified, supernova-driven, interstellar medium

2006

We report on a study of interstellar turbulence driven by both correlated and isolated supernova explosions. We use three-dimensional hydrodynamic models of a vertically stratified interstellar medium run with the adaptive mesh refinement code Flash at a maximum resolution of 2 pc, with a grid size of 0.5 × 0.5 × 10 kpc. Cold dense clouds form even in the absence of self-gravity due to the collective action of thermal instability and supersonic turbulence. Studying these clouds, we show that it can be misleading to predict physical properties such as the star formation rate or the stellar initial mass function using numerical simulations that do not include self-gravity of the gas. Even if all the gas in turbulently Jeans unstable regions in our simulation is assumed to collapse and form stars in local freefall times, the resulting total collapse rate is significantly lower than the value consistent with the input supernova rate. The amount of mass available for collapse depends on scale, suggesting a simple translation from the density PDF to the stellar IMF may be questionable. Even though the supernova-driven turbulence does produce compressed clouds, it also opposes global collapse. The net effect of supernova-driven turbulence is to inhibit star formation globally by decreasing the amount of mass unstable to gravitational collapse.

On the supernova heating of the intergalactic medium

Monthly Notices of the Royal Astronomical Society, 2000

We present estimates of the energy input from supernovae (SNe) into the intergalactic medium using (i) recent measurements of Si and Fe abundances in the intracluster medium (ICM), and (ii) self-consistent gasdynamical simulations that include processes of cooling, star formation, SNe feedback and a multiphase model of the interstellar medium. We estimate the energy input from observed abundances using two different assumptions: (i) spatial uniformity of metal abundances in the ICM, and (ii) radial abundance gradients. We show that these two cases lead to energy input estimates which are different by an order of magnitude, highlighting a need for observational data on large-scale abundance gradients in clusters. Our analysis indicates that the SNe energy input can be important for the heating of the entire ICM (providing energy of ,1 keV per particle) only if the ICM abundances are uniform and the efficiency of the gas heating by SN explosions is close to 100 per cent e SN < 1; implying that all of the initial kinetic energy of the explosion goes into heating of the ICM).