Toward first-principle simulations of galaxy formation: I. How should we choose star-formation criteria in high-resolution simulations of disk galaxies?

Abstract

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, nth affect structures of the galactic gaseous/stellar disk. We employed 10 6-107 particles to resolve a cold (T < 100K) and dense (nH > 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 nth. High-n th models with nth= 100cm-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 10-30pc. In low-nth models with nth = 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-nth models. A high-resolution simulation with high-nth 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-nth 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-nth 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 (nH ∼ 1cm-3) to the star-forming regions (nH ≳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-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modeling of star formation. © 2008. Astronomical Society of Japan.