Toward First-Principle Simulations of Galaxy Formation: I. How Should We Choose Star-Formation Criteria in High-Resolution Simulations of Disk Galaxies?

We performed three-dimensional N-body/SPH simulations to study how mass resolution and other model parameters, such as the star-formation efficiency parameter, C-* and the threshold density for star formation, n(th) affect structures of the galactic gaseous/stellar disk. We employed 10(6)-10(7) particles to resolve a cold (T < 100 K) and dense (n(H) > 100cm(-3)) phase as well as diffuse, hot phases. We found that structures of the interstellar medium (ISM) and the distribution of young stars were sensitive to the assumed values of n(th). High-n(th) models with n(th) = 100 cm(-3) yielded clumpy multi-phase features in the ISM. Young stars were distributed in a thin disk, of which the half-mass scale height was 10-30 pc. In low-n(th) models with n(th) = 0.1 cm(-3), which is usually employed in cosmological galaxy-formation simulations, the gas disk appears to be smoother and the stellar disk is found to be several-times thicker than the high-n(th) models. A high-resolution simulation with high-n(th) is necessary to reproduce the complex structure of the gas disk. The global star-formation properties of galaxies, such as the star-formation history, in low-n(th) models are similar to those in high-n(th) models when we tune the value of C-* so that they reproduce the observed relation between the surface gas density and the surface star-formation rate density. We however emphasize that high-n(th) models automatically reproduce the relation, regardless of the values of C-*. The ISM structure, phase distribution and distributions of young star-forming regions are quite similar in runs with different values of C-*. We found that the timescale of the flow from the reservoir (n(H) similar to 1 cm(-3)) to the star-forming regions (n(H) greater than or similar to 100cm(-3)) is about five-times as long as the local dynamical time, and this evolution timescale is independent of the value of C-*. The use of a high-n(th) criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modeling of star formation.