Global dynamical evolution of the ISM in star forming galaxies

Abstract

In star forming disk galaxies, matter circulation between stars and the interstellar gas, and, in particular the energy input by random and clustered supernova explosions, determine the dynamical and chemical evolution of the ISM, and hence of the galaxy as a whole. Using a 3D MHD code with adaptive mesh refinement developed for this purpose, we have investigated the rôle of magnetized matter circulation between the gaseous disk and the surrounding galactic halo. Special emphasis has been put on the effect of the magnetic field with respect to the volume and mass fractions of the different ISM "phases", the relative importance of ram, thermal and magnetic pressures, and whether the field can prevent matter transport from the disk into the halo. The simulations were performed on a grid with an area of 1 kpc2, centered on the solar circle, extending 10 kpc perpendicular to the galactic disk with a resolution as high as 1.25 pc. The simulations were run for a time scale of 400 Myr, sufficiently long to avoid memory effects of the initial setup, and to allow for a global dynamical equilibrium to be reached in case of a constant energy input rate. The main results of our simulations are: (i) The T 103 K gas is mainly concentrated in shock compressed layers, exhibiting the presence of high density clouds with sizes of a few parsecs and T 200 K. These structures are formed in regions where several large scale streams of convergent flow (driven by SNe) occur. They have lifetimes of a few free-fall times, are filamentary in structure, tend to be aligned with the local field and are associated with the highest field strengths; (ii) the magnetic field has a high variability and it is largely uncorrelated with the density, suggesting that it is driven by superalfvenic inertial motions; (iii) ram pressure controls the flow for 200 105.5 K) in contrast is controlled by the thermal pressure, since magnetic field lines are swept towards the dense compressed walls; (iv) up to 49% of the mass in the disk is concentrated in the classical thermally unstable regime 200