[论文解读] 3D Lyman-alpha radiation transfer. III. Constraints on gas and stellar properties of z~3 Lyman break galaxies (LBG) and implications for high-z LBGs and Lyman-alpha emitters(LAEs)

Anne Verhamme, D. Schaerer+2 more|ArXiv.org|May 23, 2008

Galaxies: Formation, Evolution, Phenomena参考文献 96被引用 160

一句话总结

本研究利用三维Lyman-alpha辐射转移模型，约束了z~3 Lyman break星系（LBGs）的气体与恒星性质，表明观测到的Lyα线轮廓和等效宽度主要由H i柱密度与尘埃消光（E(B-V) ~ 0.05–0.2）决定。关键结果为：大多数LBGs的本征Lyα等效宽度为50–100 Å，其差异源于辐射转移与尘埃效应，解释了LBGs与Lyα发射体（LAEs）之间的重叠现象，以及Lyα强度随红移和SFR的变化趋势。

ABSTRACT

The Aim of our study is to understand the variety of observed Lyman-alpha (Lya) line profiles and strengths in Lyman Break Galaxies (LBGs) and Lya emitters (LAEs), the physical parameters governing them, and hence deriving constraints on the gas and dust content and stellar populations of these objects. Using our 3D Lya radiation transfer code including gas and dust (Verhamme et al. 2006), we fit 11 LBGs from the FORS Deep Field with redshifts between 2.8 and 5 observed by Tapken et al. (2007). A simple geometry of a spherically expanding shell of HI is adopted. RESULTS : The variety of observed Lya profiles is successfully reproduced. Most objects show outflow velocities of 150-200 km/s; two objects are most likely quasi-static. The radial HI column density ranges from NH=2.10^{19} to 7.10^{20} cm^{-2}. Our Lya profile fits yield values of E(B-V)~0.05-0.2 for the gas extinction. We find indications for a dust-to-gas ratio higher than the Galactic value, and for a substantial scatter. The escape fraction of Lya photons is found to be determined primarily by the extinction, and a simple fit formula is proposed. Intrinsic EW(Lya)~50-100 Angstroms are found for 8/11 objects, as expected for stellar populations forming constantly over long periods (> 10-100 Myr). In three cases we found indications for younger populations. Correlations between the observed EW(Lya) and other observables such as FWHM(Lya), E(B-V),SFR(UV) etc, are reproduced. We also show that there is a clear overlap between LBGs and LAEs. Radiation transfer and dust effects explain the increase of the LAE/LBG ratio, and a higher percentage of LBGs with strong Lya emission with increasing redshift. [shortened]

研究动机与目标

• 理解高红移Lyman break星系（LBGs）与Lyα发射体（LAEs）中观测到的Lyα线轮廓与强度多样性的物理起源。
• 通过详细的辐射转移建模，约束z~3 LBGs中的气体与尘埃含量、星族性质及外流特性。
• 确定Lyα等效宽度、FWHM、消光与SFR之间的观测相关性是否源于内在差异，或由辐射转移效应引起。
• 评估LBGs与LAEs之间的关系，特别是z~3时它们观测性质的重叠现象。
• 检验在修正辐射转移与尘埃效应后，Lyα等效宽度是否可作为可靠的SFR示踪指标。

提出的方法

• 将三维Lyα辐射转移代码应用于FORS深空场中的11个z~3 LBGs，红移范围为2.8–5。
• 假设采用球对称、膨胀的H i壳层几何结构，以模拟Lyα转移与尘埃消光。
• 辐射转移过程包括共振散射、尘埃吸收（以E(B-V)为自由参数）及Lyα光子的非各向同性逃逸。
• 通过拟合观测到的Lyα线轮廓与等效宽度，将本征Lyα等效宽度作为自由参数进行模型拟合。
• 模型引入尘埃-气体比的变化，并检验其是否偏离银河系尘埃律。
• 推导出Lyα逃逸分数与E(B-V)关系的简单经验拟合公式。

实验结果

研究问题

• RQ1 哪些物理参数——特别是H i柱密度与尘埃消光——决定了z~3 LBGs中观测到的Lyα线轮廓与等效宽度的多样性？
• RQ2 辐射转移与尘埃效应在多大程度上解释了LBGs与LAEs中Lyα性质、FWHM、消光与SFR之间观测相关性？
• RQ3 LBGs的本征Lyα等效宽度与其观测值相比如何？这对恒星形成历史有何启示？
• RQ4 z~3时LBGs与LAEs之间的关系是什么？观测到的重叠现象及随红移增加的LAE/LBG比是否可由辐射转移与尘埃效应解释？
• RQ5 若已知本征Lyα等效宽度，是否可利用观测到的Lyα等效宽度推断E(B-V)？

主要发现

• 通过采用球对称外流的三维辐射转移模型，成功再现了z~3 LBGs中观测到的Lyα线轮廓与强度的多样性。
• 大多数LBGs表现出150–200 km s⁻¹的外流速度，其中两个天体与准静态条件一致。
• H i柱密度范围为N_Hi ~ 2×10¹⁹至7×10²⁰ cm⁻²，尘埃消光（E(B-V)）范围为~0.05至0.2，且存在尘埃-气体比高于银河系的证据。
• Lyα逃逸分数主要由尘埃消光决定，提出了一种简单公式，用于在已知本征等效宽度时关联EW_obs与E(B-V)。
• 发现11个LBGs中有8个的本征Lyα等效宽度约为50–100 Å，与10–100 Myr的连续恒星形成一致，另有三个天体暗示更年轻的星族。
• 在z~3时，LBGs与LAEs之间存在明显重叠：约20–25%的LBGs（其EW(Lyα)_{obs} > 20 Å）与R_AB = 25.5 mag以上的LAEs匹配，且辐射转移与尘埃效应可自然解释随红移增加的LAE/LBG比。