Millimeter dust continuum emission revealing the true mass of giant molecular clouds in the Small Magellanic Cloud
Millimeter dust continuum emission revealing the true mass of giant molecular clouds in the Small Magellanic Cloud
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毫米尘埃连续发射揭示了小麦哲伦星云中巨分子云的真实质量
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
F. Rantakyrö
中科院分区：
文献类型：
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作者：
C. Bot;C. Bot;F. Boulanger;Mónica Rubio;F. Rantakyrö
上下文CO观测是迄今为止追踪外部星系中分子气体的最佳方法，但在低金属丰度环境中，由于CO分子的光解离增强，推断的气体质量可能会被大大低估。因此，CO观测可能会错过H2的大包络。目标。目前，CO数据立方体的运动学信息被用来估计分子云的维里质量和追踪分子云的总质量。毫米粉尘排放也可用作致密气体示踪剂，并可揭示缺乏CO的H2包层。这些不同的示踪剂必须在不同的环境中进行比较。方法.这项研究比较了两个GMC样本中的维里质量和从毫米辐射推导出的质量：我们银河系中的局部分子云（10 4 −10 5 M ²），以及最近的低金属量矮星系之一小麦哲伦云（SMC）中的等效物。结果在我们的银河系中，从毫米（FIRAS）发射推断的质量估计与从伽马射线分析推断的质量一致，因此可以追踪云的总质量。维里质量系统大于（平均两倍）从毫米尘埃排放的质量估计。这种差异随着质量的增加而减小，并且在以前的研究中已经报道过。SMC巨型分子云的情况并非如此：从SIMBA毫米观测中推导出的分子云质量系统地高于SEST CO观测中的维里质量（平均为尘埃与气体比和尘埃发射率保守值的两倍）。所观察到的过量不能用尘埃性质的任何合理变化来解释。采用维里定理的一般形式，我们表明SMC云中的磁场强度为1015 μG可以为云提供额外的支持，并解释观察到的差异。结论.我们的结论是SMC分子云的质量到目前为止被低估。磁压力可能对它们的支持有很大的贡献。
Context. CO observations have been the best way so far to trace molecular gas in external galaxies, but in low metallicity environments the gas mass deduced could be largely underestimated due to enhanced photodissociation of the CO molecule. Large envelopes of H2 could therefore be missed by CO observations. Aims. At present, the kinematic information of CO data cubes are used to estimate virial masses and trace the total mass of the molecular clouds. Millimeter dust emission can also be used as a dense gas tracer and could unveil H2 envelopes lacking CO. These different tracers must be compared in different environments. Methods. This study compares virial masses to masses deduced from millimeter emission, in two GMC samples: the local molecular clouds in our Galaxy (10 4 −10 5 M� ), and their equivalents in the Small Magellanic Cloud (SMC), one of the nearest low metallicity dwarf galaxies. Results. In our Galaxy, mass estimates deduced from millimeter (FIRAS) emission are consistent with masses deduced from gamma ray analysis and therefore trace the total mass of the clouds. Virial masses are systematically larger (twice on average) than mass estimates from millimeter dust emission. This difference decreases toward high masses and has been reported in previous studies. This is not the case for SMC giant molecular clouds: molecular cloud masses deduced from SIMBA millimeter observations are systematically higher (twice on average for conservative values of the dust to gas ratio and dust emissivity) than the virial masses from SEST CO observations. The observed excess cannot be accounted for by any plausible change of dust properties. Taking a general form for the virial theorem, we show that a magnetic field strength of ∼15 µG in SMC clouds could provide additional support for the clouds and explain the difference observed. Conclusions. We conclude that masses of SMC molecular clouds have so far been underestimated. Magnetic pressure may contribute significantly to their support.