Wide-field CO isotopologue emission and the CO-to-H 2 factor across the nearby spiral galaxy M101
Wide-field CO isotopologue emission and the CO-to-H 2 factor across the nearby spiral galaxy M101
复制标题
邻近螺旋星系 M101 的宽场 CO 同位素体发射和 CO-to-H 2 因子
DOI：
10.1051/0004-6361/202245718
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发表时间：
2023
影响因子：
6.5
通讯作者：
Chiang, I-Da
中科院分区：
文献类型：
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作者：
den Brok, Jakob S.;Bigiel, Frank;Chastenet, Jérémy;Sandstrom, Karin;Leroy, Adam;Usero, Antonio;Schinnerer, Eva;Rosolowsky, Erik W.;Koch, Eric W.;Chiang, I-Da
在河外研究中，一氧化碳(CO)排放是星际介质(ISM)中最广泛使用的大分子气体示踪剂。CO-to-H转化系数α12CO(1−0)将观测到的CO排放量与总分子气体质量联系起来。然而，没有一个单一的公式完美地描述了α12CO(1−0)作为金属丰度、分子气体不透明度、线激发和其他因素的函数在星系内和跨星系的所有环境中的变化。利用绘制在附近星系的CO及其同位素的谱线观测，我们可以限制分子气体条件，并将它们与α12CO(1−0)的变化联系起来。在这里，我们提出了新的，宽视场(10 × 10 arcmin2)IRAM30米望远镜1毫米和3毫米线观测12CO，13CO，和C18O跨越附近的，宏伟的设计，螺旋星系M101。单从CO同位素谱线比分析，我们发现选择性核合成和不透明度的变化是整个银河系谱线发射变化的主要驱动因素。在进一步的分析步骤中，我们使用不同的方法估计了α12CO(1−0)，包括(I)通过从远红外发射获得的尘埃质量表面密度作为总气体表面密度的独立示踪剂，以及(Ii)基于局部热平衡的测量使用光学稀薄的13CO(1-0)强度。我们得到了银河系盘面上⟨α12co(1 − 0)⟩=4.4kpc± 0.9M⊙PC−2(K Km S−1)−1的平均值，向2kpc中心区方向减小了10倍。相反，我们发现整个磁盘上基于LTE的α12co(1−0)值比基于灰尘的结果低2-3倍。考虑到α12co(1−0)的变化，我们发现银河系中心的分子气体耗尽时间显著减少了10倍。总之，如果不考虑α12co(1−0)的变化，我们的结果对通常导出的标度关系是有意义的，例如低估了肯尼库特施密特定律的斜率。
Carbon monoxide (CO) emission constitutes the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H2conversion factor,α12CO(1−0), links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation ofα12CO(1−0)across all environments within and across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using spectral line observations of CO and its isotopologues mapped across a nearby galaxy, we can constrain the molecular gas conditions and link them to a variation inα12CO(1−0). Here, we present new, wide-field (10 × 10 arcmin2) IRAM 30-m telescope 1 mm and 3 mm line observations of12CO,13CO, and C18O across the nearby, grand-design, spiral galaxy M101. From the CO isotopologue line ratio analysis alone, we find that selective nucleosynthesis and changes in the opacity are the main drivers of the variation in the line emission across the galaxy. In a further analysis step, we estimatedα12CO(1−0)using different approaches, including (i) via the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) local thermal equilibrium (LTE) based measurements using the optically thin13CO(1–0) intensity. We find an average value of ⟨α12CO(1 − 0)⟩ = 4.4 ± 0.9M⊙pc−2(K km s−1)−1across the disk of the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-basedα12CO(1−0)values are lower by a factor of 2–3 across the disk relative to the dust-based result. Accounting forα12CO(1−0)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, ifα12CO(1−0)variations are not accounted for.