星爆星系NGC 253中的光解化学足迹
Photodissociation chemistry footprints in the Starburst galaxy NGC 253
摘要：
我们报告的PDR分子示踪剂，即HOC+，CO+的第一次检测，并确认检测到的PDR示踪剂HCO对星爆星系NGC 253，声称主要是由冲击加热和在早期阶段的演变比M 82，原型河外PDR。我们的CO+检测遭受显着的混合到一组过渡的13CH3OH，试探性地检测到第一次在河外星际介质。这些物种是有效地形成在高紫外线照射的外层分子云，在后期阶段的核恒星爆发中观察到的M 82。我们推导出的这些分子的分子丰度比与在M 82中发现的分子丰度比非常相似。这有力地支持了这样一种观点，即这些分子正在追踪与NGC 253核区域的星暴相关的PDR成分。与化学模型的预测PDR的比较表明，所观察到的分子比跟踪外层的紫外线照射的云高达两个数量级的视觉消光。化学模型，其中包括粮食形成和光解的HNCO，支持的情况下，光为主的化学作为一个解释所观察到的物种的丰度。从这个比较中，我们得出结论，在NGC 253的分子云更大的质量和更大的柱密度比M82，预期从两个星系的星爆的演化阶段。
英文摘要：
We report the first detection of PDR molecular tracers, namely HOC+, and CO+, and confirm the detection of the also PDR tracer HCO towards the starburst galaxy NGC 253, claimed to be mainly dominated by shock heating and in an earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our CO+ detection suffers from significant blending to a group of transitions of 13CH3OH, tentatively detected for the first time in the extragalactic interstellar medium. These species are efficiently formed in the highly UV irradiated outer layers of molecular clouds, as observed in the late stage nuclear starburst in M 82. The molecular abundance ratios we derive for these molecules are very similar to those found in M 82. This strongly supports the idea that these molecules are tracing the PDR component associated with the starburst in the nuclear region of NGC 253. A comparison with the predictions of chemical models for PDRs shows that the observed molecular ratios are tracing the outer layers of UV illuminated clouds up to two magnitudes of visual extinction. Chemical models, which include grain formation and photodissociation of HNCO, support the scenario of a photo-dominated chemistry as an explanation to the abundances of the observed species. From this comparison we conclude that the molecular clouds in NGC 253 are more massive and with larger column densities than those in M 82, as expected from the evolutionary stage of the starbursts in both galaxies.
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