Spiral shocks and the formation of molecular clouds in a two-phase medium

We extend recent numerical results on molecular cloud formation in spiral galaxies by including a multiphase medium. The addition of a hot phase of gas enhances the structure in the cold gas, and significantly increases the fraction of molecular hydrogen that is formed when the cold gas passes through a spiral shock. The difference in structure is reflected in the mass power spectrum of the molecular clouds, which is steeper for the multiphase calculations. The increase in molecular gas occurs as the addition of a hot phase leads to higher densities in the cold gas. In particular, cold gas is confined in clumps between the spiral arms and retains a higher molecular fraction. Unlike the single-phase results, molecular clouds are present in the inter-arm regions for the multiphase medium. However, the density of the inter-arm molecular hydrogen is generally below that which can be reliably determined from CO measurements. We therefore predict that for a multiphase medium, there will be low-density clouds containing cold atomic and molecular hydrogen, which are potentially entering the spiral arms.