通过Local Group的红外尘埃发射的CO-to-H2转换因子
The CO-to-H2 Conversion Factor From Infrared Dust Emission Across the Local Group
摘要：
我们估计的转换因子CO排放到H2的质量，α_CO，在五个本星系群星系，跨越大约一个数量级的金属量- M31，M33，大麦云（LMC），NGC 6822，和小麦哲伦云（SMC）。我们沿着视线从红外（IR）发射的灰尘质量模型，然后解决的α_CO，最好允许一个单一的气尘比（delta_GDR）来描述每个系统。这种方法仍然敏感的CO-暗信封的H2周围的分子云。在M 31、M 33和LMC中，我们发现α_CO \approx 3-9 M_sun pc^-2（K km s^-1）^-1，在不确定性范围内与银河系的值一致。在我们的样本中，两个金属丰度最低的星系NGC 6822和SMC（12 + log（O/H）\approx 8.2和8.0）表现出更高的α_CO。我们的最佳估计值是α_NGC 6822 \approx 30 M_sun/pc^-2（K km s^-1）^-1和α_SMC \approx 70 M_sun/pc^-2（K km s-1）-1。这些结果与转换因子变为CO是12 + log（O/H）\sim 8.4 - 8.2附近金属度的强函数一致。我们赞成的解释，减少灰尘屏蔽导致的主导地位的无CO信封分子云低于此金属。
英文摘要：
We estimate the conversion factor relating CO emission to H2 mass, alpha_CO, in five Local Group galaxies that span approximately an order of magnitude in metallicity - M31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the alpha_CO that best allows a single gas-to-dust ratio (delta_GDR) to describe each system. This approach remains sensitive to CO-dark envelopes of H2 surrounding molecular clouds. In M 31, M 33, and the LMC we find alpha_CO \approx 3-9 M_sun pc^-2 (K km s^-1)^-1, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log(O/H) \approx 8.2 and 8.0), exhibit a much higher alpha_CO. Our best estimates are α_NGC6822 \approx 30 M_sun/pc^-2 (K km s^-1)^-1 and α_SMC \approx 70 M_sun/pc^-2 (K km s-1)-1. These results are consistent with the conversion factor becoming CO a strong function of metallicity around 12 + log(O/H) \sim 8.4 - 8.2. We favor an interpretation where decreased dust-shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.
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