宇宙学SPH模拟中的分子氢调节恒星形成
Molecular hydrogen regulated star formation in cosmological SPH simulations
摘要：
观测表明，恒星的形成（SF）与分子氢（H $_2$）的存在密切相关。因此，在宇宙学背景下研究它对星系形成的影响是很重要的。在目前的工作中，我们跟踪的H $_2$的质量分数在我们的宇宙学平滑粒子流体动力学（SPH）代码GADGET-3使用的平衡分析模型Krumholz等。该模型允许我们调节恒星形成在我们的模拟由本地丰富的H $_2$，而不是总的冷气体密度，自然引入恒星形成的金属丰度的依赖。我们研究了基于平衡H $_2$的SF模型对星系人口性质的影响，如星晕质量比（SHMR）、重子分数、宇宙星形成率密度（SFRD）、星系特定SFR、星系恒星质量函数（GSMF）和Kennicutt-Schmidt（KS）关系.我们的工作比以前的工作的优势是有一个大的模拟星系的宇宙学体积从高红移到$z=0$的样本。我们发现，低质量晕与M $_{DM}<10^{10.5}$ M $_\odot$是不太有效的产生恒星的H $_2$为基础的SF模型在$z\geq6$，这使得模拟更好地与观测估计的SHMR和GSMF在低质量端。这是特别明显的减少，在M $_\star\leq10^{8}$ M $_\odot$在GSMF的低质量星系的数量。在H $_2$运行中，在高$z$时，整体SFRD也会降低，这导致在低红移时SFRD略高，这是因为在以后的时间里，有更多的气体可用于恒星形成。这种新的H $_2$模型能够自然地再现$z=0$处的经验KS关系，而不需要手动设置其归一化，并且总体上它似乎比先前的基于压力的SF模型具有更多的优势。
英文摘要：
It has been shown observationally that star formation (SF) correlates tightly with the presence of molecular hydrogen (H$_2$). Therefore it would be important to investigate its implication on galaxy formation in a cosmological context. In the present work, we track the H$_2$ 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 H$_2$ rather than the total cold gas density, and naturally introduce the dependence of star formation on metallicity. We investigate implications of the equilibrium H$_2$-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$_{DM}<10^{10.5}$ M$_\odot$ are less efficient in producing stars in the H$_2$-based SF model at $z\geq6$, 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 M$_\star\leq10^{8}$ M$_\odot$ in the GSMF. The overall SFRD is also reduced at high-$z$ in the H$_2$ run, which results in slightly higher SFRD at low-redshift due to more abundant gas available for star formation at later times. This new H$_2$ 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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