Full-disc 13CO(1–0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H2 conversion factor
Full-disc 13CO(1–0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H2 conversion factor
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DOI：
10.1093/mnras/sty059
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发表时间：
2018-01
影响因子：
4.8
通讯作者：
D. Cormier;F. Bigiel;M. Jiménez-Donaire;A. Leroy;M. Gallagher;A. Usero;K. Sandstrom;A. Bolatto;A. Hughes;C. Kramer;M. Krumholz;David S. Meier;E. Murphy;J. Pety;E. Rosolowsky;E. Schinnerer;A. Schruba;K. Śliwa;F. Walter
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文献类型：
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作者：
D. Cormier;F. Bigiel;M. Jiménez-Donaire;A. Leroy;M. Gallagher;A. Usero;K. Sandstrom;A. Bolatto;A. Hughes;C. Kramer;M. Krumholz;David S. Meier;E. Murphy;J. Pety;E. Rosolowsky;E. Schinnerer;A. Schruba;K. Śliwa;F. Walter
一氧化碳 (CO) 提供有关星系分子气体特性的重要信息。虽然 $^{12}$CO 已成为广泛的目标，但 $^{13}$CO 等同位素体具有光学厚度较小的优势，并且最近可以在整个星系盘上进行观测。我们使用 IRAM 30-m 望远镜提供了一个全面的新数据集，其中包含来自 EMPIRE 巡天的 9 个附近螺旋星系的完整圆盘的 IRAM 30-m 观测数据，空间分辨率为 $\sim$1.5kpc。 $^{13}$CO(1-0) 映射到 $0.7-1r_{25}$ 并在整个地图中以高信噪比进行检测。我们分析 $^{12}$CO(1-0)-to-$^{13}$CO(1-0) 比率 ($\Re$) 作为半乳心半径和其他参数的函数，例如 $^{12}$CO(2-1)-to-$^{12}$CO(1-0) 强度比、70 比 160$\mu$m 通量密度比、恒星形成速率表面密度、恒星形成效率，以及 CO 到 H$_2$ 的转换因子。我们发现 $\Re$ 在星系内部和星系之间最多变化 2 倍，中值为 11，并且星系中心的变化比盘中的变化更大。我们认为，光学深度效应（最有可能是由于扩散/致密气体混合物的变化）是对观察到的 $\Re$ 变化的有利解释，而丰度变化也可能发挥作用。我们计算了空间解析的 $^{13}$CO(1-0) 到 H$_2$ 转换因子，并找到了 $1.0\times10^{21}$ cm$^{-2}$ 的平均值（此 http URL^{-1}$ 在我们的样本上，标准偏差为 2 倍。我们发现 $^{13}$CO(1-0) 似乎不能很好地预测由于存在较大的扩散相，它可能与正常的星系盘不同，但它可能是比星系中心的 $^{12}$CO(1-0) 更好的质量示踪剂，因为星系中心的致密气体比例较大。
Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While $^{12}$CO has been targeted extensively, isotopologues such as $^{13}$CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new dataset of $^{13}$CO(1-0) observations with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from the EMPIRE survey at a spatial resolution of $\sim$1.5kpc. $^{13}$CO(1-0) is mapped out to $0.7-1r_{25}$ and detected at high signal-to-noise throughout our maps. We analyse the $^{12}$CO(1-0)-to-$^{13}$CO(1-0) ratio ($\Re$) as a function of galactocentric radius and other parameters such as the $^{12}$CO(2-1)-to-$^{12}$CO(1-0) intensity ratio, the 70-to-160$\mu$m flux density ratio, the star-formation rate surface density, the star-formation efficiency, and the CO-to-H$_2$ conversion factor. We find that $\Re$ varies by a factor of 2 at most within and amongst galaxies, with a median value of 11 and larger variations in the galaxy centres than in the discs. We argue that optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, are favored explanations for the observed $\Re$ variations, while abundance changes may also be at play. We calculate a spatially-resolved $^{13}$CO(1-0)-to-H$_2$ conversion factor and find an average value of $1.0\times10^{21}$ cm$^{-2}$ (this http URL^{-1}$ over our sample with a standard deviation of a factor of 2. We find that $^{13}$CO(1-0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than $^{12}$CO(1-0) in the galaxy centres where the fraction of dense gas is larger.