[论文解读] Proximate Molecular Quasar Absorbers: Excess of damped H2 systems at zabs~zQSO in SDSS DR14

P. Noterdaeme, S. A. Balashev+5 more|arXiv (Cornell University)|May 6, 2019

Astrophysics and Star Formation Studies参考文献 70被引用 5

一句话总结

本研究在SDSS DR14中，于类星体附近约1000 km s⁻¹范围内识别出81个受阻的H₂吸收系统，揭示了与中间吸收体相比，近类星体H₂系统存在4–5倍的过剩。其中一半系统表现出Ly-α发射漏光，表明存在致密、紧凑的云团，可能与类星体宿主星系或星系群环境相关，H₂激发有助于打破对紫外场强度和密度约束的混淆。

ABSTRACT

We present results from a search for strong H2 absorption systems proximate to quasars (zabs~zem) in the Sloan Digital Sky Survey (SDSS) Data Release 14. The search is based on the Lyman-Werner band signature of damped H2 absorption lines without any prior on the associated metal or neutral hydrogen content. This has resulted in the detection of 81 systems with log N(H2)~19-20 located within a few thousand km/s from the quasar. Compared to a control sample of intervening systems, this implies an excess of proximate H2 systems by about a factor of 4 to 5. The incidence of H2 systems increases steeply with decreasing relative velocity, reaching an order of magnitude higher than expected from intervening statistics at Delta_v<1000 km/s. The most striking feature of the proximate systems compared to the intervening ones is the presence of Ly-alpha emission in the core of the associated damped HI absorption line in about half of the sample. This puts constraints on the relative projected sizes of the absorbing clouds to those of the quasar line emitting regions. Using the SDSS spectra, we estimate the HI, metal and dust content of the systems, which are found to have typical metallicities of one tenth Solar, albeit with a large spread among individual systems. We observe trends between the fraction of leaking Ly-alpha emission and the relative absorber-quasar velocity as well as with the excitation of several metal species, similar to what has been seen in metal-selected proximate DLAs. With the help of theoretical HI-H2 transition relations, we show that the presence of H2 helps to break the degeneracy between density and strength of the UV field as main sources of excitation and hence provides unique constraints on the possible origin and location of the absorbing clouds. We suggest that most of these systems originate from galaxies in the quasar group. [truncated]

研究动机与目标

• 研究类星体附近（z_abs ≈ z_QSO）的受阻H₂吸收系统的发生率与特性，重点关注其与类星体的空间和运动学关系。
• 确定近类星体H₂系统与中间Damped Lya吸收体（DLAs）在金属丰度、尘埃含量和激发状态方面的差异。
• 利用H₂激发和Ly-α发射漏光作为诊断工具，评估这些系统的起源与物理条件。
• 利用H₂丰度与H i–H₂转变理论，约束云团的密度、距类星体的距离以及潜在的反馈效应。
• 探讨活动星系核反馈在类星体环境中塑造含H₂云团的形成与生存中的作用。

提出的方法

• 在SDSS DR14类星体光谱中，通过Lyman-Werner带特征对H₂吸收进行盲搜，不预先假设金属或H i含量。
• 在类星体红移Δv < 3000 km s⁻¹范围内，识别出81个近类星体H₂系统，其N(H₂) ≈ 10¹⁹–10²⁰ cm⁻²。
• 将近类星体系统的发生率与中间Damped Lya吸收体（DLAs）的对照样本进行比较，以量化过剩程度及速度依赖性。
• 从吸收轮廓中测量H i柱密度、金属丰度、尘埃消光以及精细结构线（如Si ii、C ii）的激发状态。
• 利用理论H i–H₂转变关系（Sternberg et al. 2014）基于H₂丰度推断云团的密度与距类星体的距离。
• 分析Ly-α发射漏光比例，作为吸收体部分覆盖类星体电离区的代理指标。

实验结果

研究问题

• RQ1 在类星体附近Δv < 3000 km s⁻¹范围内，受阻H₂吸收系统的发生率与中间系统相比如何？
• RQ2 Ly-α发射漏光在H i吸收线核心的存在，如何约束吸收云团的大小与几何结构？
• RQ3 近类星体H₂系统的典型金属丰度、尘埃含量与激发状态为何？与中间Damped Lya吸收体（DLAs）有何差异？
• RQ4 H₂激发能否打破对吸收云团中紫外场强度与气体密度之间混淆？
• RQ5 基于物理诊断，近类星体H₂系统最可能的起源是什么——类星体宿主星系、星系群成员，还是喷流或吸积气体？

主要发现

• 在类星体Δv < 3000 km s⁻¹范围内，N(H₂) ≈ 10¹⁹–10²⁰ cm⁻²的近类星体H₂系统相比中间系统存在4–5倍的过剩。
• H₂系统发生率随相对速度减小而急剧上升，在Δv < 1000 km s⁻¹时高出预期值达10倍。
• 约50%的近类星体H₂系统表现出Ly-α发射漏光的特征，表明存在致密、紧凑的吸收云团，且部分覆盖了类星体的发射线区。
• 近类星体系统表现出约0.1倍太阳金属丰度的典型值，但存在较大分散，且Si ii与C ii精细结构线的激发程度增强。
• H₂激发数据有助于打破对紫外场强度与气体密度之间混淆，表明靠近类星体的云团需具有高密度（n_H ~ 10⁴–10⁵ cm⁻³）才能形成H₂。
• 大多数系统可能与类星体星系群中的星系相关，尽管一小部分可能源自类星体宿主星系或喷流/吸积气体。