Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM
Dense cloud cores revealed by CO in the low metallicity dwarf galaxy WLM
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DOI：
10.1038/nature14901
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发表时间：
2015-09-10
期刊：
影响因子：
64.8
通讯作者：
Cigan, Phil
中科院分区：
文献类型：
--
作者：
Rubio, Monica;Elmegreen, Bruce G.;Cigan, Phil
了解恒星的诞生需要观测它们形成的云。这些云是致密的和自引力的，在所有现有的观测中，它们是分子的，其中H-2是主要的物质，一氧化碳（CO）是最好的可用示踪剂（1，2）。当碳和氧的丰度低于氢的丰度时，尘埃的不透明度也很低，如原始星系和本地矮不规则星系（3），CO形成缓慢，容易被破坏，因此它很难积聚在稠密的云中（4）。在这里，我们报告了在本星系群矮不规则星系沃尔夫-伦德马克-梅洛特（WLM）（5）中对CO云的干涉观测，该星系的金属丰度是太阳值的13%（6，7），比以前的CO检测阈值低50%。与周围的原子和H-2包层相比，这些云很小，但它们具有银河系中CO云的典型密度和柱密度。正常的CO密度解释了为什么矮不规则星系中形成的星星团与巨螺旋星系中的星星团具有相似的密度。低质量的云表明，这些星团也将是低质量的，除非发生一些星系级的压缩，例如来自宇宙云或其他星系的撞击。如果像人们普遍认为的那样，银河系晕中的大质量贫金属球状星团是在矮星系中形成的，那么它们很可能是由这样的撞击引发的。
Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations they are molecular, with H-2 the dominant species and carbon monoxide (CO) the best available tracer(1,2). When the abundances of carbon and oxygen are low compared with that of hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies(3), CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds(4). Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM)(5), which has a metallicity that is 13 per cent of the solar value(6,7) and 50 per cent lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H-2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.