The interstellar medium and star formation of galactic disks. I. Interstellar medium and giant molecular cloud properties with diffuse far-ultraviolet and cosmic-ray backgrounds

We present a series of adaptive mesh refinement hydrodynamic simulations of flat rotation curve galactic gas disks, with a detailed treatment of the interstellar medium (ISM) physics of the atomic to molecular phase transition under the influence of diffuse farultraviolet (FUV) radiation fields and cosmic-ray backgrounds. We explore the effects of different FUV intensities, including a model with a radial gradient designed to mimic the Milky Way. The effects of cosmic rays, including radial gradients in their heating and ionization rates, are also explored. The final simulations in this series achieve 4 pc resolution across the similar to 20 kpc global disk diameter, with heating and cooling followed down to temperatures of similar to 10 K. The disks are evolved for 300 Myr, which is enough time for the ISM to achieve a quasi-statistical equilibrium. In particular, the mass fraction of molecular gas is stabilized by similar to 200 Myr. Additional global ISM properties are analyzed. Giant molecular clouds (GMCs) are also identified and the statistical properties of their populations are examined. GMCs are tracked as the disks evolve. GMC collisions, which may be a means of triggering star cluster formation, are counted and their rates are compared with analytic models. Relatively frequent GMC collision rates are seen in these simulations, and their implications for understanding GMC properties, including the driving of internal turbulence, are discussed.