The Volume Densities of Giant Molecular Clouds in M81
The Volume Densities of Giant Molecular Clouds in M81
复制标题
M81 中巨型分子云的体积密度
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
P. C. van der Kruit
中科院分区：
文献类型：
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作者：
J. Heiner;R. Allen;B. Emonts;P. C. van der Kruit
M81中年轻星团附近的H I特征被确认为巨型分子云(GMC)的光解离面，年轻恒星就是从GMC中形成的。这些特征的HI柱密度呈现微弱的趋势，从R=3.7kpc内无法检测到的值，并迅速增加到R−7.5kpc附近约3×1021 cm≈2的值。这一趋势类似于该星系中H i的径向平均分布，这意味着在整个M81期间，GMC的面积覆盖因子为≈0.21。我们的候选PDR样本的入射紫外光通量G0沿径向减小。光解-重整过程的一个简单的平衡模型将入射紫外光通量、HI柱密度和相对尘埃含量的观测值联系起来，从而允许对GMC中的总气体密度做出独立的估计。在GMC中，这种气体将主要是分子氢。得到了1<；n<；200 cm−3的体积密度，几何平均值为17 cm−3。这些值与银河系中GMC的密度相似，但比以前用类似方法得到的M101的密度略低。在M81的GMC中，分子密度的低值将导致CO(1-0)跃迁的低能级碰撞激发，这与在M81的盘中观察到的CO(1-0)发射的很低的表面亮度是一致的。
H I features near young star clusters in M81 are identified as the photodissociated surfaces of giant molecular clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 × 1021 cm −2 near R ≈ 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of ≈0.21 for GMCs throughout M81. The incident UV fluxes G0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1 < n < 200 cm −3 are derived, with a geometric mean of 17 cm−3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81.