Numerical Modeling of the ISM in the Galactic Center and Disks

The probability distribution functions (PDF) of density of the interstellar medium (ISM) in galactic disks and global star formation rate (SFR) are discussed. 3-D hydrodynamic simulations show that the PDFs in globally stable, inhomogeneous ISM in galactic disks are well fitted by a single log-normal function over a wide density range. The dispersion of the log-normal PDF (LN-PDF) is larger for more gas-rich systems, whereas the characteristic density of LN-PDF, for which the volume fraction becomes the maximum, is not sensitive for the gas mass. Using the LN-PDF, we give a generalized version of Schmidt-Kennicutt law, i.e. SFR as a function of average gas density, a critical local density for star formation, and star formation efficiency. We find that the observed SFR in normal and starburst galaxies are well-fitted by the theoretical SFR in a wide range of the global gas density (10 –10⁴ M⊙pc^–2). Star formation efficiency (SFE) for high density gas (n>10³ cm^–3) may be intrinsically different between starburst and normal galaxies.