Molecular cloud formation and magnetic fields in spiral galaxies

We present ongoing hydrodynamic and MHD simulations of molecular cloud formation in spiral galaxies. The hydrodynamic results show the formation of molecular gas clouds where spiral shocks compress atomic gas to high densities. The spiral shocks also produce structure in the spiral arms, provided the gas is cold (< 1000 K). When both hot and cold components of the ISM are modeled, this structure is enhanced. Properties such as the clump mass spectra and spatial distribution will be compared from clouds identified in these simulations. In particular the multiphase simulations predict the presence of much more interarm molecular gas than when a single phase is assumed. We also discuss very recent results from galactic-scale MHD calculations. From observational comparisons of the magnetic and thermal pressure, magnetic fields are expected to be a major factor in explaining the dynamics of the ISM, from kpc scales to those of star formation. We describe the difference in structure of the spiral arms, and the evolution of the global magnetic field for a range of field strengths.