[论文解读] Physical conditions in the gas phases of the giant HII region LMC-N11 unveiled by Herschel - I. Diffuse [CII] and [OIII] emission in LMC-N11B

V. Lebouteiller, D. Cormier+11 more|arXiv (Cornell University)|Sep 26, 2012

Astrophysics and Star Formation Studies参考文献 66被引用 45

一句话总结

本研究利用赫歇尔/PACS对大麦哲伦星云-N11B HII区的远红外图像，解耦了[ C II] 157 μm与[ O III] 88 μm发射的起源。通过与光学示踪剂及光致电离模型对比，发现95%的[ C II]发射源自光子主导区（PDRs），而[ O III] 88 μm则追踪了扩展的、低密度电离气体，其中高达50%的冷却过程发生在以[ O I] 63 μm主导的致密PDRs中。结果表明，多环芳烃（PAHs）比总红外发射更能有效追踪PDR中的气体加热，其光电效率约为7%。

ABSTRACT

(Abridged) The Magellanic Clouds provide a nearby laboratory for metal-poor dwarf galaxies. The low dust abundance enhances the penetration of UV photons into the interstellar medium (ISM), resulting in a relatively larger filling factor of the ionized gas. Furthermore, there is likely a hidden molecular gas reservoir probed by the [CII]157um line. We present Herschel/PACS maps in several tracers, [CII], [OI]63um,145um, [NII]122um, [NIII]57um, and [OIII]88um in the HII region N11B in the Large Magellanic Cloud. Halpha and [OIII]5007A images were used as complementary data to investigate the effect of dust extinction. Observations were interpreted with photoionization models to infer the gas conditions and estimate the ionized gas contribution to the [CII] emission. Photodissociation regions (PDRs) are probed through polycyclic aromatic hydrocarbons (PAHs). We first study the distribution and properties of the ionized gas. We then constrain the origin of [CII]157um by comparing to tracers of the low-excitation ionized gas and of PDRs. [OIII] is dominated by extended emission from the high-excitation diffuse ionized gas; it is the brightest far-infrared line, ~4 times brighter than [CII]. The extent of the [OIII] emission suggests that the medium is rather fragmented, allowing far-UV photons to permeate into the ISM to scales of >30pc. Furthermore, by comparing [CII] with [NII], we find that 95% of [CII] arises in PDRs, except toward the stellar cluster for which as much as 15% could arise in the ionized gas. We find a remarkable correlation between [CII]+[OI] and PAH emission, with [CII] dominating the cooling in diffuse PDRs and [OI] dominating in the densest PDRs. The combination of [CII] and [OI] provides a proxy for the total gas cooling in PDRs. Our results suggest that PAH emission describes better the PDR gas heating as compared to the total infrared emission.

研究动机与目标

• 确定大麦哲伦星云低金属丰度HII区N11B中弥漫远红外（FIR）冷却线发射的起源。
• 利用低临界密度、光学薄的示踪剂（如[ O III] 88 μm和[ N II] 122 μm）约束电离气体的填充因子。
• 区分电离气体、PDRs和中性气体对[ C II] 157 μm发射（低金属丰度环境中关键冷却剂）的贡献。
• 评估多环芳烃（PAHs）与尘埃（通过总红外辐射TIR示踪）在PDR中加热气体的相对重要性。

提出的方法

• 赫歇尔/PACS对N11B区域进行了多条FIR线的空间分辨映射，包括[ C II] 157 μm、[ O I] 63 μm与145 μm、[ N II] 122 μm、[ N III] 57 μm以及[ O III] 88 μm。
• 利用光学Hα和[ O III] 5007 Å图像评估尘埃消光，并与FIR线发射（特别是[ O III] 88 μm）进行比较。
• 应用光致电离模型解释[ O III] 88 μm发射，假设所有已确认的O型星均有贡献，估算电离气体密度和电离参数。
• 利用斯皮兹勒空间望远镜对多环芳烃（PAHs）的观测探测PDR条件，因为PAHs可示踪PDR中的气体加热。
• 将[ C II] + [ O I] 63 μm发射与PAH和总红外（TIR）发射进行比较，评估其相关性，并推断气体冷却与加热效率。
• 通过比较气体冷却（通过[ C II] + [ O I]）与加热示踪剂（PAH和TIR）的比值（[ C II] + [ O I])/PAH与([ C II] + [ O I])/TIR，估算PAH和尘埃颗粒的光电效率。

实验结果

研究问题

• RQ1 N11B中[ C II] 157 μm发射有多少比例起源于PDRs，又有多少比例起源于电离气体？
• RQ2 [ O III] 88 μm线如何追踪HII区中扩展的、低密度电离气体？这揭示了电离气体的填充因子如何？
• RQ3 与总红外发射相比，PAHs与气体冷却（通过[ C II] + [ O I]）的相关性如何？这对气体加热机制有何启示？
• RQ4 尘埃消光在遮蔽电离气体发射方面起到什么作用？它如何影响观测到的[ O III] 88 μm与光学[ O III] 5007 Å线的比值？
• RQ5 从[ O III] 88 μm和[ N III]/[ N II]比值推断的电离气体物理条件（密度、电离参数）与O型星的预期值相比如何？

主要发现

• [ O III] 88 μm线是N11B中最亮的FIR线，亮度约为[ C II] 157 μm的四倍，主要来自低密度、高激发电离气体的扩展发射。
• 对恒星团LH 10和区域W2的尘埃消光估计为A_V ≈ 1，表明约一半的电离气体发射在光学波段被遮蔽，尘埃质量与此消光一致。
• [ O III] 88 μm发射的空间分布与所有已确认O型星的电离作用一致，前提是大尺度上电离气体密度≤16 cm⁻³。
• 在N11B中，95%的[ C II] 157 μm发射源自PDRs，仅有最多15%来自LH 10附近恒星团的电离气体。
• 发现[ C II] + [ O I] 63 μm与PAH发射之间存在紧密相关性，其中[ C II]在稀薄PDR中主导冷却，而[ O I] 63 μm在最致密PDR区域贡献高达50%。
• PAHs在PDR中主导气体加热，光电效率约为7%，显著高于从总红外发射推断的约0.6%效率，表明PAHs比以尘埃为主导的TIR更能有效追踪PDR加热。