Robert Thompson, Kentaro Nagamine, Jason Jaacks, Jun-Hwan Choi
It has been shown observationally that star formation (SF) correlates tightly with the presence of molecular hydrogen (H2). Therefore it would be important to investigate its implication on galaxy formation in a cosmological context. In the present work, we track the H2 mass fraction within our cosmological smoothed particle hydrodynamics (SPH) code GADGET-3 using an equilibrium analytic model by Krumholz et al. This model allows us to regulate the star formation in our simulation by the local abundance of H2 rather than the total cold gas density, and naturally introduce the dependence of star formation on metallicity. We investigate implications of the equilibrium H2-based SF model on galaxy population properties, such as the stellar-to-halo mass ratio (SHMR), baryon fraction, cosmic star formation rate density (SFRD), galaxy specific SFR, galaxy stellar mass functions (GSMF), and Kennicutt-Schmidt (KS) relationship. The advantage of our work over the previous ones is having a large sample of simulated galaxies in a cosmological volume from high-redshift to z=0. We find that low-mass halos with M_DM<10^{10.5} Msun are less efficient in producing stars in the H2-based SF model at z>=6, which brings the simulations to a better agreement with observational estimates of SHMR and GSMF at the low-mass end. This is particularly evident by a reduction in the number of low-mass galaxies at Mstar<=10^{8} Msun in the GSMF. The overall SFRD is also reduced at high-z in the H2 run, which results in slightly higher SFRD at low-redshift due to more abundant gas available for star formation at later times. This new H2 model is able to reproduce the empirical KS relationship at z=0 naturally without the need for setting its normalization by hand, and overall it seems to have more advantages than the previous pressure-based SF model.
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http://arxiv.org/abs/1301.0063
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