In most of numerical simulations of spiral galaxy formation,
mass/spatial resolution is ~ 105-6M๏ and kpc or sub-kpc,
therefore inhomogeneous structure of the ISM in galaxies is not
resolved. This is the most serious defect in simulating star
formation and its feedback during galaxy formation/evolution. Here we
show an intrinsic structures of the ISM using 3-D high resolution
hydrodynamic simulations of galactic disks. We 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. 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 also introduce our new project, “Project Milky Way”, in which we
aim to resolve properly the cold, dense ISM, as found in above
simulations, by ultra-high resolution during galaxy formation. We are
planning to construct a special cluster for simulating formation of
“Milky Way” using the next generation GRAPE.