[论文解读] Confirmation of double peaked Lyman-alpha emission at z=6.593: Witnessing a galaxy directly contributing to the reionisation of the Universe

Jorryt Matthee, David Sobral+4 more|arXiv (Cornell University)|May 29, 2018

Galaxies: Formation, Evolution, Phenomena参考文献 124被引用 34

一句话总结

该论文通过深度VLT/X-SHOOTER光谱观测和多波段测光，确认COLA1为红移z=6.593处首个观测到的双峰Lyman-alpha发射体。其窄而不对称的谱线轮廓，蓝-红流量比为0.31，且H i柱密度较低（N_Hi ≈ 10^17.0 cm⁻²），表明存在显著的电离光子逃逸（15–30%），为这类高红移星系在宇宙再电离过程中发挥积极作用提供了直接证据。

ABSTRACT

Distant luminous Lyman-alpha emitters (LAEs) are excellent targets for spectroscopic observations of galaxies in the epoch of reionisation (EoR). We present deep high-resolution (R=5000) VLT/X-SHOOTER observations, along with an extensive collection of photometric data of `COLA1', a proposed double peaked LAE at z=6.6 (Hu et al. 2016). We rule out that COLA1's emission line is an [OII] doublet at z=1.475 on the basis of i) the asymmetric red line-profile and flux ratio of the peaks (blue/red=$0.31\pm0.03$) and ii) an unphysical [OII]/Ha ratio ([OII]/Ha > 22). We show that COLA1's observed B-band flux is explained by a faint extended foreground LAE, for which we detect Lya and [OIII] at z=2.142. We thus conclude that COLA1 is a real double-peaked LAE at z=6.593, the first discovered at z>6, confirming the result from Hu et al. (2016). COLA1 is UV luminous (M$_{1500}=-21.6\pm0.3$), has a high equivalent width (EW$_{0}$~120 Å) and very compact Lya emission (r$_{50} = 0.3$ kpc). Relatively weak inferred Hb+[OIII] line-emission from Spitzer/IRAC indicates an extremely low metallicity of Z<1/20 Z$_{\odot}$ or reduced strength of nebular lines due to high escape of ionising photons. The small Lya peak separation of $220\pm20$ km/s implies a low HI column density and an ionising photon escape fraction of ~15-30 %, providing the first direct evidence that such galaxies contribute actively to the reionisation of the Universe at z>6. Based on simple estimates, we find that COLA1 could have provided just enough photons to reionise its own ~0.3 pMpc bubble, allowing the blue Lya line to be observed. However, we also discuss alternative scenarios explaining the detected double peaked nature of COLA1. Our results show that future high-resolution observations of statistical samples of double peaked LAEs at z>5 are a promising probe of the occurrence of ionised regions around galaxies in the EoR.

研究动机与目标

• 确认COLA1——一个z≈6.6的候选双峰Lyman-alpha发射体——为真实的高红移发射线星系。
• 通过流量比和线比诊断，排除z=1.475处[O ii]发射的其他解释。
• 确定COLA1的本征性质，包括Lyα光度、等效宽度、紧凑性以及电离光子逃逸分数。
• 通过估算其电离泡的大小，评估COLA1在电离星际介质过程中的贡献。
• 确立高红移LAE中双峰Lyα轮廓作为探测电离区与再电离时期星系反馈的探针。

提出的方法

• 利用VLT/X-SHOOTER进行深度、高分辨率（R=5000）光谱观测，获得COLA1的高信噪比光谱。
• 分析Lyα谱线轮廓的不对称性及流量比（蓝/红 = 0.31±0.03），该结果与z=1.475处[O ii]双峰发射不一致。
• 利用COSMOS和UltraVISTA的测光数据，识别出一个位于前方的z=2.142 LAE，其通过Lyα和[O iii]发射贡献了B波段的流量。
• 采用五参数壳层模型拟合Lyα轮廓，推断H i柱密度和电离光子逃逸分数。
• 结合UV光度（M₁₅₀₀ = -21.6±0.3）和Spitzer/IRAC颜色，推断低金属丰度（Z<1/20 Z⊙）以及由于光子逃逸导致的星云发射线减弱。
• 基于光子预算和复合 timescales 的简单假设，估算COLA1周围电离泡的大小。

实验结果

研究问题

• RQ1 COLA1在z=6.593处的双峰Lyα发射是否为真实的高红移Lyman-alpha发射线，还是如z=1.475处[O ii]双峰发射等人为假象？
• RQ2 COLA1中Lyα发射的本征轮廓及物理条件（如H i柱密度、电离光子逃逸分数）为何？
• RQ3 COLA1是否足以电离其周围足够大的区域，使Lyα的蓝峰能够逃逸？该电离泡的大小是多少？
• RQ4 COLA1中Lyα发射的紧凑性与高等效宽度如何与其在再电离过程中的作用相关？
• RQ5 双峰Lyα轮廓在高红移LAE中的意义是什么，作为探测再电离时期电离区与星系反馈的探针？

主要发现

• 确认COLA1为真实存在的双峰Lyman-alpha发射体，红移z_Lyα,red = 6.593，基于流量比和线比诊断，排除了z=1.475处[O ii]发射的可能性。
• Lyα发射的光度为L_Lyα = 4.1±0.2×10⁴³ erg s⁻¹，静止系等效宽度为EW₀,Lyα = 120⁺⁵⁰₋₄₀ Å，表明发射非常强烈。
• Lyα发射极为紧凑，半光半径为r₅₀,Lyα = 0.33⁺⁰.⁰⁷₋₀.⁰⁴ kpc，与紧凑的恒星形成区一致。
• 峰值间距为220±20 km s⁻¹，蓝峰流量仅为红峰的31%，表明散射不对称且H i柱密度较低（N_Hi = 10¹⁷.⁰±⁰.³ cm⁻²）。
• 推断的电离光子逃逸分数约为15–30%，上限可达约50%，依据与LyC泄漏的相关性及谱线轮廓建模。
• COLA1可能已电离了约0.3 pMpc（2.3 cMpc）的周围区域，与观测到的蓝Lyα峰可检测性一致，但可能需要外部光源维持电离泡。