Braine Jonathan, Herpin Fabrice
Observatoire de Bordeaux, UMR 5804, CNRS/INSU, B.P. 89, F-33270 Floirac, France.
Nature. 2004 Nov 18;432(7015):369-71. doi: 10.1038/nature03054.
Knowledge about the outermost portions of galaxies is limited owing to the small amount of light coming from them. It is known that in many cases atomic hydrogen (H I) extends well beyond the optical radius. In the centres of galaxies, however, molecular hydrogen (H2) usually dominates by a large factor, raising the question of whether H2 is also abundant in the outer regions. Here we report the detection of emission from carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius of the nearby galaxy NGC 4414. The host molecular clouds probably formed in the regions of relatively high H i column density and in the absence of spiral density waves. The relative strength of the lines from the two lowest rotational levels indicates that both the temperature and density of the H2 are quite low compared to conditions closer to the centre. The inferred surface density of the molecular material continues the monotonic decrease from the inner regions. We conclude that although molecular clouds can form in the outer region of this galaxy, there is little mass associated with them.
由于来自星系最外层的光线很少，我们对其了解有限。已知在许多情况下，原子氢（H I）延伸的范围远远超出光学半径。然而，在星系中心，分子氢（H₂）通常占主导地位，这就引发了一个问题：H₂在星系外部区域是否也很丰富。在此，我们报告在邻近星系NGC 4414的光学半径之外探测到一氧化碳（CO）的发射，CO是H₂最丰富的示踪剂。宿主分子云可能形成于H I柱密度相对较高且没有螺旋密度波的区域。来自两个最低转动能级的谱线相对强度表明，与靠近星系中心的情况相比，H₂的温度和密度都相当低。推断出的分子物质表面密度从星系内部区域开始持续单调下降。我们得出结论，尽管这个星系的外部区域能够形成分子云，但与之相关的质量很少。
