星系中中面压力与分子氢的关系：环境依赖性
The relation between mid-plane pressure and molecular hydrogen in galaxies: Environmental dependence
摘要：
分子氢（H2）是我们银河系中为恒星形成提供燃料的冷而致密的气体库的主要成分。虽然H2丰度最终是由星际介质（ISM）中小尺度上的物理过程来调节的，但观测结果显示，附近螺旋星系中氢分子与原子的比例与其圆盘中平面的压力之间存在密切的相关性。这种经验关系已被用来预测H2丰度的星系与潜在的非常不同的ISM条件，如金属缺乏星系在高红移。在这里，我们测试这种方法的有效性，通过研究的压力- H2关系的ISM环境参数的依赖性。为此，我们明确遵循H2的形成和破坏在一套不同的ISM参数的星系的流体动力学模拟。我们发现，压力- H2的关系自然出现在我们的模拟各种尘埃气体比或强度的星际辐射场在ISM。固定的尘埃-气体比和紫外线辐射场的值在太阳附近的结果在公平的协议与附近的星系中观察到的关系，大致太阳的金属丰度。然而，压力- H2关系的参数（斜率和归一化）以系统的方式随ISM特性而变化。一个特别强烈的趋势是，随着ISM的尘气比的降低，关系的正常化程度降低。我们表明，这种趋势和其他属性的压力- H2关系是自然的后果，从原子到分子氢与气体表面密度的过渡。
英文摘要：
Molecular hydrogen (H2) is the primary component of the reservoirs of cold, dense gas that fuel star formation in our galaxy. While the H2 abundance is ultimately regulated by physical processes operating on small scales in the interstellar medium (ISM), observations have revealed a tight correlation between the ratio of molecular to atomic hydrogen in nearby spiral galaxies and the pressure in the mid-plane of their disks. This empirical relation has been used to predict H2 abundances in galaxies with potentially very different ISM conditions, such as metal-deficient galaxies at high redshifts. Here, we test the validity of this approach by studying the dependence of the pressure -- H2 relation on environmental parameters of the ISM. To this end, we follow the formation and destruction of H2 explicitly in a suite of hydrodynamical simulations of galaxies with different ISM parameters. We find that a pressure -- H2 relation arises naturally in our simulations for a variety of dust-to-gas ratios or strengths of the interstellar radiation field in the ISM. Fixing the dust-to-gas ratio and the UV radiation field to values measured in the solar neighborhood results in fair agreement with the relation observed in nearby galaxies with roughly solar metallicity. However, the parameters (slope and normalization) of the pressure -- H2 relation vary in a systematical way with ISM properties. A particularly strong trend is the decrease of the normalization of the relation with a lowering of the dust-to-gas ratio of the ISM. We show that this trend and other properties of the pressure -- H2 relation are natural consequences of the transition from atomic to molecular hydrogen with gas surface density.
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