arXiv 2302.030442023-02-06astro-ph.GA

近邻旋涡星系M101中的大视场CO同位素分子发射与CO-to-H$_2$转换因子

Wide-field CO isotopologue emission and the CO-to-H$_2$ factor across the nearby spiral galaxy M101

Jakob S. den Brok, Frank Bigiel, Jérémy Chastenet, Karin Sandstrom, Adam Leroy, Antonio Usero, Eva Schinnerer, Erik W. Rosolowsky, Eric W. Koch, I-Da Chiang, As… 展开作者

Jakob S. den Brok, Frank Bigiel, Jérémy Chastenet, Karin Sandstrom, Adam Leroy, Antonio Usero, Eva Schinnerer, Erik W. Rosolowsky, Eric W. Koch, I-Da Chiang, Ashley T. Barnes, Johannes Puschnig, Toshiki Saito, Ivana Bešlić, Melanie Chevance, Daniel A. Dale, Cosima Eibensteiner, Simon Glover, María J. Jiménez-Donaire, Yu-Hsuan Teng, Thomas G. Williams

备注：: Accepted for publication in A&A, 25 pages, 15 figures
发表信息：: A&A 676, A93 (2023)
DOI：: 10.1051/0004-6361/202245718

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AI总结：

本研究利用IRAM 30米对M101的CO同位素谱线观测，约束分子气体条件并估算$α_{\rm CO}$，发现其中心下降10倍，进而显著改变分子气体耗竭时间与Kennicutt-Schmidt关系。

摘要：

一氧化碳（CO）发射是河外研究中应用最广泛的星际介质（ISM）主体分子气体示踪物。CO-to-H$_2$转换因子$α_{\rm CO}$将观测到的CO发射与总分子气体质量联系起来。然而，没有任何单一方案能够完美描述$α_{\rm CO}$在星系间各类环境中随金属丰度、分子气体不透明度、谱线激发及其他因素发生的变化。利用CO及其同位素分子的分辨谱线观测，我们可以约束分子气体条件，并将其与转换因子的变化联系起来。我们展示了IRAM 30米望远镜在1mm和3mm波段对近邻星系M101中$^{12}$CO、$^{13}$CO和C$^{18}$O的新谱线观测。基于CO同位素分子谱线比，我们发现选择性核合成和不透明度变化是该星系内谱线发射变化的主要驱动因素。此外，我们采用不同方法估算了$α_{\rm CO(1-0)}$，包括：（i）由远红外发射导出的尘埃质量面密度，作为总气体面密度的独立示踪物；（ii）利用光学薄的$^{13}$CO(1-0)强度进行基于LTE的测量。我们得到整个星系的平均$α_{\rm CO}=4.4{\pm}0.9\rm\\,M_\odot\\,pc^{-2}(K\\,km\\,s^{-1})^{-1}$，而在朝向2 kpc中心区域时该值下降10倍。相比之下，我们发现相对于基于尘埃的结果，基于LTE的数值在整个盘面上低2至3倍。考虑$α_{\rm CO}$的变化后，我们发现星系中心的分子气体耗竭时间显著缩短，缩短幅度达10倍。总之，我们的结果表明，如果不考虑$α_{\rm CO}$的变化，将对通常导出的标度关系产生影响，例如低估Kennicutt-Schmidt定律的斜率。

英文摘要：

Carbon monoxide (CO) emission is the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H$_2$ conversion factor, $α_{\rm CO}$, links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of $α_{\rm CO}$ across all environments across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using resolved spectral line observations of CO and its isotopologues, we can constrain the molecular gas conditions and link them to a variation in the conversion factor. We present new IRAM 30-m 1mm and 3mm line observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O} across the nearby galaxy M101. Based on the CO isotopologue line ratios, we find that selective nucleosynthesis and opacity changes are the main drivers of the variation in the line emission across the galaxy. Furthermore, we estimated $α_{\rm CO(1-0)}$ using different approaches, including (i) the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) LTE-based measurements using the optically thin $^{13}$CO(1-0) intensity. We find an average value of $α_{\rm CO}=4.4{\pm}0.9\rm\,M_\odot\,pc^{-2}(K\,km\,s^{-1})^{-1}$ across the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based values are lower by a factor of 2-3 across the disk relative to the dust-based result. Accounting for $α_{\rm CO}$ variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy's center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if $α_{\rm CO}$ variations are not accounted for.

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