小麦哲伦星云中的红外暗云？
Infrared Dark Clouds in the Small Magellanic Cloud?
摘要：
我们已经应用unsharp-masking技术的24 $μ$ m的小麦哲伦云（SMC）的图像，获得与斯皮策太空望远镜，寻找高消光区。这项技术已经被用来定位银河系中非常密集和寒冷的星际云，特别是红外暗云（IRDC）。五十五个候选区域的高消光，即高对比度区域（HCR），已被确定从生成的递减对比度图像的SMC。大多数HCR位于南部的酒吧区，主要分布在CO云的外围，但最有可能包含大量的H2。HCR在24 $μ$ m处的峰值对比度为2 - 2.5%，尺寸为8 - 14 pc。这与典型的大型银河系IRDC的大小相对应，但银河系IRDC在24 $μ$ m处比我们的HCR暗2 - 3倍。为了约束的HCR的物理性质，我们已经进行了NH3，N2 H+，HNC，HCO+，HCN的观测对HCR之一，HCR LIRS 36-EAST，使用澳大利亚望远镜紧凑阵列和Mopra单碟射电望远镜。我们没有检测到任何分子谱线发射，然而，我们对分子种类的柱密度的上限表明，HCR最有可能是中等密度的，n约为10^{3} cm-3。这个体积密度与低金属丰度环境中冷原子相的预测一致。我们认为，HCR可能是在从原子到分子为主的介质过渡的云跟踪，并可能是一个强大的方法来研究早期阶段的气体凝聚在低金属丰度星系。或者，如果HCR是由密度小于0.5 pc的分子团组成的，那么它可能是银河系IRDCs的对应物，或者是具有非常不寻常的非常小的尘埃颗粒丰度的区域。
英文摘要：
We have applied the unsharp-masking technique to the 24 $μ$m image of the Small Magellanic Cloud (SMC), obtained with the Spitzer Space Telescope, to search for high-extinction regions. This technique has been used to locate very dense and cold interstellar clouds in the Galaxy, particularly infrared dark clouds (IRDCs). Fifty five candidate regions of high-extinction, namely high-contrast regions (HCRs), have been identified from the generated decremental contrast image of the SMC. Most HCRs are located in the southern bar region and mainly distributed in the outskirts of CO clouds, but most likely contain a significant amount of H2. HCRs have a peak-contrast at 24 $μ$m of 2 - 2.5 % and a size of 8 - 14 pc. This corresponds to the size of typical and large Galactic IRDCs, but Galactic IRDCs are 2 - 3 times darker at 24 $μ$m than our HCRs. To constrain the physical properties of the HCRs, we have performed NH3, N2H+, HNC, HCO+, and HCN observations toward one of the HCRs, HCR LIRS36-EAST, using the Australia Telescope Compact Array and the Mopra single-dish radio telescope. We did not detect any molecular line emission, however, our upper limits to the column densities of molecular species suggest that HCRs are most likely moderately dense with n ~ 10^{3} cm-3. This volume density is in agreement with predictions for the cool atomic phase in low metallicity environments. We suggest that HCRs may be tracing clouds at the transition from atomic to molecule-dominated medium, and could be a powerful way to study early stages of gas condensation in low metallicity galaxies. Alternatively, if made up of dense molecular clumps < 0.5 pc in size, HCRs could be counterparts of Galactic IRDCs, and/or regions with highly unusual abundance of very small dust grains.
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