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arXiv 1607.08036astro-ph.GA
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照亮暗分子气体：H $_{2}$作为直接示踪剂

Lighting the dark molecular gas: H$_{2}$ as a direct tracer

Aditya Togi, J. D. T. Smith

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14 Figures, 6 Tables, Accepted for publication in ApJ
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摘要：

分子气体质量的可靠知识对于理解星系中恒星的形成至关重要。H $_{2}$分子在寒冷的星际介质中不能有效地发射，因此星系的分子气体含量通常使用间接示踪剂来推断。在低金属丰度和其他极端环境中，这些示踪剂可能会受到很大的偏差。本文提出了一种直接从纯中红外转动H $_{2}$辐射估算星系中分子气体总质量的新方法。通过假设H $_{2}$旋转温度的幂律分布，我们可以精确地模拟H $_{2}$激发，并通过仅改变幂律的斜率可靠地获得温暖的（$T\!\gtrsim\!100$ K）H $_{2}$气体质量。与斯皮策/IRS的典型灵敏度，我们能够直接探测的H $_{2}$内容通过旋转发射低至~80 K，占~15%的总分子气体质量的星系。通过外推拟合的幂律温度分布的校准的\emph{single}较低的截止温度，该模型还恢复了总的分子含量的一个因素内的~2.2在不同的星系样本，和一个子集的破幂律模型同样表现良好。在ULIRGs中，温暖的H $_{2}$气体的比例随着尘埃温度的升高而升高，并对$α_\mathrm{CO}$有一定的依赖性。在5个低金属量星系的样本中，范围低至12 + log [O/H]= 7.8，该模型产生的分子质量比CO所暗示的大约100倍，与其他基于尘埃质量和恒星形成耗尽时标的方法一致。这项技术为评估早期宇宙中的分子含量提供了真正的希望，而早期宇宙中的CO和尘埃方法可能会失败。

英文摘要：

Robust knowledge of molecular gas mass is critical for understanding star formation in galaxies. The H$_{2}$ molecule does not emit efficiently in the cold interstellar medium, hence the molecular gas content of galaxies is typically inferred using indirect tracers. At low metallicity and in other extreme environments, these tracers can be subject to substantial biases. We present a new method of estimating total molecular gas mass in galaxies directly from pure mid-infrared rotational H$_{2}$ emission. By assuming a power-law distribution of H$_{2}$ rotational temperatures, we can accurately model H$_{2}$ excitation and reliably obtain warm ($T\!\gtrsim\!100$ K) H$_{2}$ gas masses by varying only the power law's slope. With sensitivities typical of Spitzer/IRS, we are able to directly probe the H$_{2}$ content via rotational emission down to ~80 K, accounting for ~15% of the total molecular gas mass in a galaxy. By extrapolating the fitted power law temperature distributions to a calibrated \emph{single} lower cutoff temperature, the model also recovers the total molecular content within a factor of ~2.2 in a diverse sample of galaxies, and a subset of broken power law models performs similarly well. In ULIRGs, the fraction of warm H$_{2}$ gas rises with dust temperature, with some dependency on $α_\mathrm{CO}$. In a sample of five low metallicity galaxies ranging down to 12+log[O/H]=7.8, the model yields molecular masses up to ~100 times larger than implied by CO, in good agreement with other methods based on dust mass and star formation depletion timescale. This technique offers real promise for assessing molecular content in the early universe where CO and dust-based methods may fail.

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