The effects of metallicity, UV radiation and non-equilibrium chemistry in high-resolution simulations of galaxies
The effects of metallicity, UV radiation and non-equilibrium chemistry in high-resolution simulations of galaxies
复制标题
金属丰度、紫外线辐射和非平衡化学对星系高分辨率模拟的影响
DOI：
10.1093/mnras/stw327
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发表时间：
2015
影响因子：
4.8
通讯作者：
J. Schaye
中科院分区：
文献类型：
--
作者：
A. J. Richings;J. Schaye
我们提出了一系列恒星质量为 $10^{9} \, \rm{M}_{\odot}$ 的孤立星系的流体动力学模拟。该模型使用每个粒子 750 美元 \, \rm{M}_{\odot}$ 的分辨率，并包括对离子和分子（总共 157 种）的完全非平衡化学演化的处理，以及使用非平衡丰度自洽计算的气体冷却速率。我们将这些与使用假设化学（包括电离）平衡计算的冷却速率进行的模拟进行比较，并且我们考虑了广泛的金属丰度和紫外线辐射场，包括气体和灰尘自屏蔽的局部处方。我们发现，在金属丰度较高和辐射场较弱的情况下，恒星形成速率更高，流出更强，因为在这种条件下，气体可以更容易冷却到冷（几百开尔文）恒星形成阶段。与金属丰度和辐射场的变化相反，非平衡化学通常对总恒星形成速率或流出特性没有强烈影响。然而，它对于模拟分子流出很重要。例如，以 $> 50 \, \rm{km} \, \rm{s}^{-1}$ 流出的 H$_{2}$ 质量在非平衡状态下增强了 $\sim 20$ 倍。我们还计算了 CII 和 CO 的可观测线发射。两者在金属丰度较高时都更强，而 CII 和 CO 发射分别在辐射场较强和较弱时较高。我们发现 CII 通常不受非平衡化学的影响。然而，CO 排放量的变化系数为 $\sim 2 - 4$。这对 CO 排放和 H$_{2}$ 柱密度之间的平均 $X_{\rm{CO}}$ 转换因子（我们发现在非平衡状态下降低了 $\sim 2.3$）以及 CO 暗分子气体的比例产生了影响。
We present a series of hydrodynamic simulations of isolated galaxies with stellar mass of $10^{9} \, \rm{M}_{\odot}$. The models use a resolution of $750 \, \rm{M}_{\odot}$ per particle and include a treatment for the full non-equilibrium chemical evolution of ions and molecules (157 species in total), along with gas cooling rates computed self-consistently using the non-equilibrium abundances. We compare these to simulations evolved using cooling rates calculated assuming chemical (including ionisation) equilibrium, and we consider a wide range of metallicities and UV radiation fields, including a local prescription for self-shielding by gas and dust. We find higher star formation rates and stronger outflows at higher metallicity and for weaker radiation fields, as gas can more easily cool to a cold (few hundred Kelvin) star forming phase under such conditions. Contrary to variations in the metallicity and the radiation field, non-equilibrium chemistry generally has no strong effect on the total star formation rates or outflow properties. However, it is important for modelling molecular outflows. For example, the mass of H$_{2}$ outflowing with velocities $> 50 \, \rm{km} \, \rm{s}^{-1}$ is enhanced by a factor $\sim 20$ in non-equilibrium. We also compute the observable line emission from CII and CO. Both are stronger at higher metallicity, while CII and CO emission are higher for stronger and weaker radiation fields respectively. We find that CII is generally unaffected by non-equilibrium chemistry. However, emission from CO varies by a factor of $\sim 2 - 4$. This has implications for the mean $X_{\rm{CO}}$ conversion factor between CO emission and H$_{2}$ column density, which we find is lowered by up to a factor $\sim 2.3$ in non-equilibrium, and for the fraction of CO-dark molecular gas.