[论文解读] The ionising photon production efficiency at z~6 for Lyman-alpha emitters using JEMS and MUSE

Charlotte Simmonds, Sandro Tacchella+22 more|arXiv (Cornell University)|Mar 14, 2023

Electron and X-Ray Spectroscopy TechniquesMaterials Science被引用 3

一句话总结

本研究利用JWST JEMS中波段测光和MUSE光谱学，测量了红移z~6处30个莱曼-Alpha发射体的电离光子产生效率（ξ_ion,0）。通过使用Prospector拟合光谱能量分布（SED）并校正尘埃影响，发现其ξ_ion,0的中位数为log(ξ_ion,0) = 25.44 Hz erg⁻¹，表明这些星系在产生电离光子方面效率极高，其值略高于以往文献估计。

ABSTRACT

We study the ionising photon production efficiency at the end of the Epoch of Reionisation ($z \sim 5.4 - 6.6$) for a sample of 30 Lyman-$α$ emitters. This is a crucial quantity to infer the ionising photon budget of the Universe. These objects were selected to have reliable spectroscopic redshifts, assigned based on the profile of their Lyman-$α$ emission line, detected in the MUSE deep fields. We exploit medium-band observations from the JWST extragalactic medium band survey (JEMS) to find the flux excess corresponding to the redshifted \ha\ emission line. We estimate the ultra-violet (UV) luminosity by fitting the full JEMS photometry, along with several HST photometric points, with exttt{Prospector}. We find a median UV continuum slope of $β= -2.09^{+0.23}_{-0.21}$ for the sample, indicating young stellar populations with little-to-no dust attenuation. Supported by this, we derive $ξ_{ion,0}$ with no dust attenuation and find a median value of log$\frac{ξ_{ion,0}}{ ext{Hz erg}^{-1}} = 25.44^{+0.21}_{-0.15}$. If we perform dust attenuation corrections and assume a Calzetti attenuation law, our values are lowered by $\sim 0.1$ dex. Our results suggest Lyman-$α$ emitters at the Epoch of Reionisation have enhanced $ξ_{ion,0}$ compared to previous estimations from literature, in particular, when compared to the non-Lyman-$α$ emitting population. This initial study provides a promising outlook on the characterisation of ionising photon production in the early Universe. In the future, a more extensive study will be performed on the entire dataset provided by the JWST Advanced Deep Extragalactic Survey (JADES). Thus, for the first time, allowing us to place constraints on the wider galaxy populations driving reionisation.

研究动机与目标

• 测量高红移莱曼-Alpha发射体在z~6时的电离光子产生效率（ξ_ion,0），这是理解早期宇宙电离预算的关键参数。
• 利用多波段测光和光谱学表征这些星系的恒星种群和尘埃含量，以评估其电离光子输出。
• 评估莱曼-Alpha发射体是否比非发射星系更高效地产生电离光子，从而可能识别其作为再电离主要驱动力的可能性。
• 为未来利用完整JADES数据集的研究提供基准，从而对再电离时期星系群体施加更广泛的约束。

提出的方法

• 基于莱曼-Alpha线轮廓的光谱确认红移（z~5.4–6.6），从MUSE深场中选取了30个莱曼-Alpha发射体。
• 利用JWST JEMS巡天的中波段测光检测红移后的Hα发射线，并估算Hα光度。
• 使用Prospector SED拟合代码拟合完整的SED，整合JEMS测光和HST测光点，推导出1500 Å处的rest-frame紫外光度密度。
• 从rest-frame 1250–2600 Å区域计算紫外连续谱斜率β，以推断恒星种群年龄和尘埃消光程度。
• 通过使用Calzetti消光律，计算考虑和不考虑尘埃校正的ξ_ion,0，即电离光子产生率与紫外光度密度之比。
• 评估光谱波段中[N ii]的污染影响，并确认对z~6星系的结果影响可忽略。

实验结果

研究问题

• RQ1 z~6处莱曼-Alpha发射体的电离光子产生效率（ξ_ion,0）是多少？与以往文献估计相比有何差异？
• RQ2 这些莱曼-Alpha发射体在多大程度上受到尘埃消光影响？尘埃校正如何影响推导出的ξ_ion,0值？
• RQ3 莱曼-Alpha发射体是否比相同红移下非莱曼-Alpha发射星系更高效地产生电离光子？
• RQ4 这些星系的恒星种群特性（如紫外斜率β）如何反映其电离输出，并可能揭示其在宇宙再电离中的潜在作用？
• RQ5 JEMS与MUSE数据的结合能否为高红移星系的Hα光度和SED拟合提供稳健约束？

主要发现

• 样本的电离光子产生效率中位数为log(ξ_ion,0) = 25.44 Hz erg⁻¹，95%置信区间为+0.21至-0.15 dex。
• 紫外连续谱斜率中位数为β = -2.09，表明其恒星种群年轻且尘埃消光可忽略。
• 使用Calzetti律进行尘埃校正时，非可忽略尘埃的样本中ξ_ion,0降低约0.1 dex，但未校正值仍为主要结果。
• 推导出的ξ_ion,0值略高于涵盖z~0至z~9的文献样本，表明莱曼-Alpha发射体可能是更高效的电离源。
• ξ_ion,0最高值出现在JADES-GS+53.13859-27.79024，该源还表现出活动星系核（AGN）活动迹象，未来将进行深入研究。
• 本研究确认，对于z~6星系，JEMS测光波段中的[N ii]污染可忽略，支持Hα流量测量的可靠性。