MHD Simulations of the ISM: The Importance of the Galactic Magnetic Field on the ISM "Phases"

We have carried out 1.25 pc resolution MHD simulations of the ISM, on a Cartesian grid of $0 \leq (x,y) \leq 1$ kpc size in the galactic plane and $-10 \leq z \leq 10$ kpc into the halo, thus being able to fully trace the time-dependent evolution of the galactic fountain. The simulations show that large scale gas streams emerge, driven by SN explosions, which are responsible for the formation and destruction of shocked compressed layers. The shocked gas can have densities as high as 800 cm$^{-3}$ and lifetimes up to 15 Myr. The cold gas is distributed into filaments which tend to show a preferred orientation due to the anisotropy of the flow induced by the galactic magnetic field. Ram pressure dominates the flow in the unstable branch $10^{2}<$T$\leq 10^{3.9}$ K, while for T$\leq 100$ K (stable branch) magnetic pressure takes over. Near supernovae thermal and ram pressures determine the dynamics of the flow. Up to 80% of the mass in the disk is concentrated in the thermally unstable regime $10^{2}<$T$\leq 10^{3.9}$ K with $\sim30$ of the disk mass enclosed in the T$\leq 10^{3}$ K gas. The hot gas in contrast is controlled by the thermal pressure, since magnetic field lines are swept towards the dense compressed walls.Comment: 8 pages, 8 figures (in jpeg format) that include 2 simulations images and 6 plots. Paper accepted by the referee for publication in the proceedings of ``Magnetic fields and star formation: theory versus observations'', kluwe

The gradient of diffuse gamma-ray emission in the Galaxy

We show that the well-known discrepancy between the radial dependence of the Galactic cosmic ray (CR) nucleon distribution, as inferred most recently from EGRET observations of diffuse gamma-rays above 100 MeV, and of the most likely CR source distribution (supernova remnants, pulsars) can be explained purely by PROPAGATION effects. Contrary to previous claims, we demonstrate that this is possible, if the dynamical coupling between the escaping CRs and thermal plasma is taken into account, and thus a self-consistent GALACTIC WIND calculation is carried out. Given a dependence of the CR source distribution on Galactocentric radius, r, our numerical wind solutions show that the CR outflow velocity, V(r,z) depends both on r, and on vertical distance, z, at reference level z_C. The latter is defined as the transition boundary from diffusion to advection dominated CR transport and is therefore also a function of r. In fact, the CR escape time averaged over particle energies decreases with increasing CR source strength. Such an increase is counteracted by a reduced average CR residence time in the gas disk. Therfore pronounced peaks in the radial source distribution result in mild radial gamma-ray gradients at GeV energies, as it has been observed. This effect is enhanced by anisotropic diffusion, assuming different radial and vertical diffusion coefficients. We have calculated 2D analytic solutions of the stationary diffusion-advection equation, including anisotropic diffusion, for a given CR source distribution and a realistic outflow velocity field V(r,z), inferred from self-consistent numerical Galactic Wind simulations. At TeV energies the gamma-rays from the sources are expected to dominate the observed "diffuse" flux from the disk. Its observation should allow an empirical test of the theory presented.Comment: 23 pages, 12 figures; accepted for publication in Astronomy and Astrophysics Main Journa

ISM Simulations: An Overview of Models

Until recently the dynamical evolution of the interstellar medium (ISM) was simulated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic processes operating on different timescales. A step forward in the field comprises a multi-fluid approach taking into account the joint thermal and dynamical evolutions of the ISM gas.Comment: Overview paper (3 pages) presented by M. Avillez at the Special Session "Modern views of the interstellar medium", XXVIIIth IAU General Assembly, August 27-30, 2012, Beijing. Chin

The Milky Way's Kiloparsec Scale Wind: A Hybrid Cosmic-Ray and Thermally Driven Outflow

We apply a wind model, driven by combined cosmic-ray and thermal-gas pressure, to the Milky Way, and show that the observed Galactic diffuse soft X-ray emission can be better explained by a wind than by previous static gas models. We find that cosmic-ray pressure is essential to driving the observed wind. Having thus defined a "best-fit" model for a Galactic wind, we explore variations in the base parameters and show how the wind's properties vary with changes in gas pressure, cosmic-ray pressure and density. We demonstrate the importance of cosmic rays in launching winds, and the effect cosmic rays have on wind dynamics. In addition, this model adds support to the hypothesis of Breitschwerdt and collaborators that such a wind may help explain the relatively small gradient observed in gamma-ray emission as a function of galactocentric radius.Comment: 14 pages, 11 figures; Accepted to Ap

Dynamical Evolution of a Supernova Driven Turbulent Interstellar Medium

It is shown that a number of key observations of the Galactic ISM can be understood, if it is treated as a highly compressible and turbulent medium energized predominantly by supernova explosions (and stellar winds). We have performed extensive numerical high resolution 3D hydrodynamical and magnetohydrodynamical simulations with adaptive mesh refinement over sufficiently long time scales to erase memory effects of the initial setup. Our results show, in good agrement with observations, that (i) volume filling factors of the hot medium are modest (typically below 20%), (ii) global pressure is far from uniform due to supersonic (and to some extent superalfvenic) turbulence, (iii) a significant fraction of the mass (about 60%) in the warm neutral medium is in the thermally unstable regime (500 K < T < 5000 K), (iv) the average number density of OVI in absorption is 1.81 10^{-8} cm^{-3}, in excellent agreement with Copernicus and FUSE data, and its distribution is rather clumpy, consistent with its measured dispersion with distance