Molecular Clouds and Star Formation Rate in Disk Galaxies

We use N-body/hydrodynamic simulations of a Milky Way-like galaxy to study the physical properties of giant molecular clouds and star formation rate on different spatial scales. We confirm the previous estimates that the dark gas fraction in molecular clouds reaches about 30% by mass. We find that conversion factors for individual molecular clouds deviated by a factor of several times from the mean value for the Milky Way clouds, XCO = 2 × 1020 cm−2 (K km s−1)−1, and hence a constant XCO conversion factor cannot represent the physical properties of an individual molecular cloud or a small sample of clouds sufficiently well, but the conversion factor averaged over the whole ensemble of clouds is believed to be close to the MilkyWay value. The ∑SFRUV−∑gas$\Sigma _{{\rm{SFR}}}^{{\rm{UV}}} - \Sigma _{{\rm{gas}}}$ relation can be reproduced using UV calibration for smoothing scale compared to the size of individual star-forming complexes, ~ 0.3 − 1 kpc.