[论文解读] A translucent interstellar cloud at z=2.69: CO, H2 and HD in the line-of-sight to SDSS J123714.60+064759.5

P. Noterdaeme, P. Petitjean+4 more|arXiv (Cornell University)|Aug 3, 2010

Astrophysics and Star Formation Studies参考文献 69被引用 4

一句话总结

本研究利用VLT/X-shooter和UVES光谱仪，对类星体SDSS J123714.60+064759.5方向的高红移部分电离星际云（z=2.69）进行了CO、H₂和HD的探测。该云具有较高的分子分数（fH₂ ≈ 0.8）、较低的动能温度（T ≈ 108 K）以及强烈的氘保留（星体演化因子 ≈ 3），表明其为致密、寒冷且化学演化的环境，消光程度较低（AV ≈ 0.14）但柱密度较高，暗示此类云在测 magnitude 限制的巡天中可能被遗漏。

ABSTRACT

We present the analysis of a sub-DLA system (log N(H^0)=20.0+/-0.15, z_abs=2.69) toward SDSS J123714+064759 (z_em=2.78). Using the VLT/UVES and X-shooter spectrographs, we detect H2, HD and CO molecules in absorption with log N(H2,HD,CO)=(19.21,14.48,14.17). The overall metallicity of the system is super-solar ([Zn/H]=+0.34) and iron is highly depleted ([Fe/Zn]=-1.39), revealing metal-rich and dusty gas. The strongest H2 component does not coincide with the centre of the HI absorption. This implies that the molecular fraction in this component, f=2N(H2)/(2N(H2)+N(H^0)), is larger than the mean molecular fraction =1/4 in the system. This is supported by the detection of Cl^0 associated with this H2-component having N(Cl^0)/N(Cl^+)>0.4. Since Cl^0 is tied up to H2 by charge exchange reactions, this means that the molecular fraction in this component is not far from unity. The size of the molecular cloud is probably smaller than 1pc. Both the CO/H2=10^-5 and CO/C^0~1 ratios for f>0.24 indicate that the cloud classifies as translucent, i.e., a regime where carbon is found both in atomic and molecular form. The corresponding extinction, Av=0.14, albeit lower than the definition of a translucent sightline (based on extinction properties), is high for the observed H^0 column density. This means that intervening clouds with similar local properties but with larger column densities could be missed by current magnitude-limited QSO surveys. The excitation of CO is dominated by radiative interaction with the Cosmic Microwave Background Radiation (CMBR) and we derive Tex(CO)=10.5+0.8-0.6 K when TCMBR(z=2.69)=10.05 K is expected. The astration factor of deuterium -with respect to the primordial D/H ratio- is only about 3. This can be the consequence of accretion of unprocessed gas from the intergalactic medium onto the associated galaxy. [abridged]

研究动机与目标

• 通过遥远类星体方向的吸收谱探测并表征高红移部分电离星际云中的分子物种。
• 利用H₂、CO和HD的转动能级跃迁，确定云中的物理条件（温度、密度、分子分数）。
• 通过测量N(HD)/N(H₂)并对比原始值，评估化学演化和氘丰度。
• 评估此类分子云在当前及未来巡天中的可探测性，尤其考虑到其消光低且物理尺度小。

提出的方法

• 利用VLT/X-shooter和UVES进行高分辨率光谱观测，分析类星体光谱中H₂、CO、HD及其他物种的吸收线。
• 通过分析H₂（J=0,1）的转动能级分布，推导出动能温度和粒子密度。
• 利用C I精细结构能级估算中性氢密度（nH₀ ≈ 50–60 cm⁻³）。
• 将CO激发态与z=2.69时宇宙微波背景辐射（CMBR）温度（10.05 K）进行比较，推导出激发温度（Tex(CO) = 10.5 K）。
• 通过N(H₂)和N(H I)计算分子分数fH₂，并通过速度位移评估部分遮蔽效应。
• 利用N(H I)/nH₀和Cl I丰度的限制条件，推断物理尺度（0.05–0.6 pc），估算云的大小。

实验结果

研究问题

• RQ1 通过CO、H₂和HD吸收探测到的高红移部分电离云的物理状态（温度、密度、分子分数）如何？
• RQ2 尽管柱密度和动能温度与本地ISM相似，为何该云的CO激发温度更高？
• RQ3 该云中的氘丰度与原始值相比如何？这对气体处理和吸积过程意味着什么？
• RQ4 尽管柱密度高，为何此类分子云在测 magnitude 限制的类星体巡天中可能被遗漏？

主要发现

• 该云主成分的分子分数fH₂ ≈ 0.8，N(H₂) = 10¹⁹.²¹ cm⁻²，表明其为致密、寒冷的环境。
• 动能温度为T = 108⁺⁸⁴₋₃₃ K，由H₂ J=0和J=1转动能级推导得出；中性氢密度nH₀ ≈ 50–60 cm⁻³，由C I精细结构能级估算得出。
• CO的激发温度为Tex(CO) = 10.5⁺⁰.⁸₋₀.⁶ K，与z=2.69时CMBR温度（10.05 K）一致，为热大爆炸模型提供了有力支持。
• N(HD)/2N(H₂) = 10⁻⁵，约为本地银河系ISM中fH₂ ≈ 1/4时的10倍，与本地ISM中更高的分子分数一致。
• 氘星体演化因子仅为≈3，表明氘的处理极少，可能源于未处理的星际气体吸积。
• 该云的物理尺度被限制在0.05–0.6 pc之间，消光程度低（AV ≈ 0.14），解释了其在以往巡天中难以被探测到的原因，尽管其柱密度很高。