z~0.45的裸分子云
A Bare Molecular Cloud at z~0.45
摘要：
在Z~0.45处的弱MgII吸收线系统（W_r（2796）~0.15埃）中，沿瞄准线HE 0001 -2340的方向探测到了几种中性物种（MgI，SiI，CaI，FeI）。正如D 'Odorico（2007）所指出的，这些观测需要极端的物理条件。我们通过运行广泛的光电离模型网格，进一步限制了这个系统的性质，确定产生中性吸收的吸收云非常密集（~100-1000/cm ^3），冷（<100 K），并且具有显着的分子含量（~72-94%）。这种大小和温度的结构已经在银河系CO调查中被检测到，并且已经在湍流气体的流体动力学模拟中被预测。为了解释在所有中性和单电离化学跃迁中观察到的谱线轮廓，谱线必须遭受未解决的饱和和/或吸收体必须部分覆盖背景类星体的宽发射谱线区域。除了这个极不寻常的云外，还有三个普通的弱MgII云（密度约为0.005/cm^3，温度约为10000 K）位于同一视线范围内，速度在500 km/s以内。我们认为，“裸分子云”，这似乎是居住在星系盘外，可能有原位恒星形成，并可能演变成一个普通的弱镁吸收云。
英文摘要：
Several neutral species (MgI, SiI, CaI, FeI) have been detected in a weak MgII absorption line system (W_r(2796)~0.15 Angstroms) at z~0.45 along the sightline toward HE0001-2340. These observations require extreme physical conditions, as noted in D'Odorico (2007). We place further constraints on the properties of this system by running a wide grid of photoionization models, determining that the absorbing cloud that produces the neutral absorption is extremely dense (~100-1000/cm^3), cold (<100 K), and has significant molecular content (~72-94%). Structures of this size and temperature have been detected in Milky Way CO surveys, and have been predicted in hydrodynamic simulations of turbulent gas. In order to explain the observed line profiles in all neutral and singly ionized chemical transitions, the lines must suffer from unresolved saturation and/or the absorber must partially cover the broad emission line region of the background quasar. In addition to this highly unusual cloud, three other ordinary weak MgII clouds (within densities of ~0.005/cm^3 and temperatures of ~10000K) lie within 500 km/s along the same sightline. We suggest that the "bare molecular cloud", which appears to reside outside of a galaxy disk, may have had in situ star formation and may evolve into an ordinary weak MgII absorbing cloud.
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