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arXiv 0902.1578astro-ph.GA
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大麦云云中的分子和原子气体- I。CO检测条件

Molecular and Atomic Gas in the Large Magellanic Cloud - I. Conditions for CO Detection

T. Wong, A. Hughes, Y. Fukui, A. Kawamura, N. Mizuno, J. Ott, E. Muller, J. L. Pineda, D. E. Welty, S. Kim, Y. Mizuno, M. Murai, T. Onishi

摘要:

利用最近完成的第二次南天CO巡天,分析了探测大麦云(LMC)中CO(1-0)发射的条件。特别是,我们研究了CO积分强度和HI积分强度,峰值亮度温度和线宽之间的相关性在2.6'(~40 pc)的分辨率。我们发现,显着的HI柱密度和峰值亮度温度是必要的,但不是充分的条件CO检测,与许多地区的强HI发射不与分子云。对于给定的HI强度,即使在>200 pc的尺度上平均,CO强度的大的散射仍然存在,表明这种散射不仅仅是由于GMC附近HI向H_2的局部转化。我们集中在两种可能性来解释这种散射:要么存在空间变化的I(CO)到N(H_2)的转换因子,或一个显着的分数的原子气体是不参与分子云的形成。一氧化碳排放量被抑制大HI线宽的微弱趋势支持第二种假设,只要大线宽可能是温暖的HI指示,并呼吁从碰撞HI流形成分子云的可能性的问题。我们还发现,分子气体与原子气体的比值与恒星表面密度没有显著的相关性(或反相关性),但与中面流体静压P_h有相关性。后一种相关性在很大程度上反映了在高HI柱密度下CO检测的可能性增加。

英文摘要:

We analyze the conditions for detection of CO(1-0) emission in the Large Magellanic Cloud (LMC), using the recently completed second NANTEN CO survey. In particular, we investigate correlations between CO integrated intensity and HI integrated intensity, peak brightness temperature, and line width at a resolution of 2.6' (~40 pc). We find that significant HI column density and peak brightness temperature are necessary but not sufficient conditions for CO detection, with many regions of strong HI emission not associated with molecular clouds. The large scatter in CO intensities for a given HI intensity persists even when averaging on scales of >200 pc, indicating that the scatter is not solely due to local conversion of HI into H_2 near GMCs. We focus on two possibilities to account for this scatter: either there exist spatial variations in the I(CO) to N(H_2) conversion factor, or a significant fraction of the atomic gas is not involved in molecular cloud formation. A weak tendency for CO emission to be suppressed for large HI linewidths supports the second hypothesis, insofar as large linewidths may be indicative of warm HI, and calls into question the likelihood of forming molecular clouds from colliding HI flows. We also find that the ratio of molecular to atomic gas shows no significant correlation (or anti-correlation) with the stellar surface density, though a correlation with midplane hydrostatic pressure P_h is found when the data are binned in P_h. The latter correlation largely reflects the increasing likelihood of CO detection at high HI column density.

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