Global dynamical evolution of the ISM in star forming galaxies - I. High resolution 3D simulations: Effect of the magnetic field

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 < T ≤ 10^5.5 K. For T ≤ 200 K magnetic pressure dominates, while the hot gas (T > 10^5.5 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 < T ≤ 10^3.9 K with ∼65% of the warm neutral medium (WNM) mass enclosed in the 500 ≤ T ≤ 5000 K gas, consistent with recent observations; (v) the volume filling factors of the different temperature regimes depend sensitively on the existence of the duty cycle between the disk and halo, acting as a pressure release mechanism for the hot phase in the disk. We find that in general gas transport into the halo in 3D is not prevented by an initial disk parallel magnetic field, but only delayed initially, for as long as it is needed to punch holes into the thick magnetized gas disk. The mean volume filling factor of the hot phase in the disk is similar in HD and MHD (the latter with a total field strength of 4.4 μG) runs, amounting to ∼17−21% for the Galactic supernova rate.