[论文解读] The impact of faint AGN discovered by JWST on reionization

Shikhar Asthana, Martin G. Haehnelt+5 more|Repository@Nottingham (University of Nottingham)|Sep 23, 2024

Solar and Space Plasma Dynamics被引用 5

一句话总结

本研究对 sherwood-relics 水动力学模拟进行后处理，使用基于 GPU 的辐射传输代码，以评估 JWST 发现的微弱AGN对氢和氦再电离的贡献，比较 QSO 辅助和仅 QSO 场景与仅星系模型。

ABSTRACT

The relative contribution of emission from stellar sources and accretion onto supermassive black holes to reionization has been brought into focus again by the apparent high abundance of faint Active Galactic Nuclei (AGN) at $4\lesssim z\lesssim11$ uncovered by JWST. We investigate here the contribution of these faint AGN to hydrogen and the early stages of helium reionization using the GPU-based radiative transfer code ATON-HE by post-processing a cosmological hydrodynamical simulation from the sherwood-relics suite of simulations. We study four models: two galaxy-only late-end reionization models, a QSO-assisted and a QSO-only model. In the QSO-assisted model, 1% of the haloes host AGN, with AGN luminosities scaled to contribute 17% of the total hydrogen-ionizing emissivity. In the QSO-only model, quasars account for all the hydrogen-ionizing emissivity, with 10% of the haloes hosting AGN. The SED of AGN is assumed to be a power-law with $α=-1.7$ each with a 10 Myr lifetime. All models are calibrated to the observed mean Lyman-$α$ forest transmission at $5\lesssim z\lesssim6.2$. The QSO-assisted model requires an emissivity similar to the galaxy-only models and fits the observed distribution of the Lyman-$α$ optical depths well. The QSO-only model is inconsistent with the observed Lyman-$α$ optical depths distribution, and produces excessively high IGM temperatures at $z\lesssim 5$ due to an early onset of HeII reionization, unless the escape fraction of HeII-ionizing photons is assumed to be low. Our results suggest that a modest AGN contribution to reionization aligns with the Lyman-$α$ forest data, whereas an AGN dominated scenario is difficult to reconcile.

研究动机与目标

• 评估在现有再电离模型中，微弱AGN如何影响氢再电离和早期的He II 再电离。
• 将仅星系、QSO 辅助和仅 QSO 的离子化源模型与 Lyman-α 森林观测进行比较。
• 将模型校准至观测到的在 5 < z < 6.2 的平均 Lyman-α 森林透射率，以确保与数据的一致性。

提出的方法

• 使用 ATON-HE GPU 辐射传输对 sherwood-relics 宇宙学流体动力学模拟进行后处理。
• 将AGN建模为寄居于质量较大的晕中 1–10% 的瞬态源，寿命为 10 Myr。
• 在 912 Å 以蓝端的幂律谱，指数 α = -1.7；分成三个电离能箱。
• 将总电离辐射强度进行标定，使其匹配 5 < z < 6.2 时观测到的平均 Lyman-α 森林透射率。
• 比较四种模型：基线的仅星系模型、寡头式的仅星系模型、QSO 辅助（AGN 对 H 离子化发射率贡献 17%，1% 的晕承载 AGN），以及 QSO-only（AGN 主导所有 H 离子化发射率，10% 的晕承载 AGN）。
• 计算电离辐射发射率的演化、IGM 温度、He II/He III 的分数、平均自由程，以及 Lyman-α 光学深度分布。

Figure 1: Left: the assumed black-body spectrum for galaxies and power-law spectrum for the QSO-assisted and QSO-only models in this work. The galaxy spectrum is mono-frequency. Right: the black-body spectrum for the galaxies-only simulations, i.e. the fiducial and Oligarchic models. The galaxy spec

实验结果

研究问题

• RQ1 如 JWST所推断的那样，纳入微弱AGN对氢再电离的时间线及其性质有何影响？
• RQ2 QSO 辅助或 QSO-only 的AGN人群是否能如星系仅模型那样或更好地再现 Lyman-α 森林观测？
• RQ3 纳入微弱AGN如何影响 He II 再电离历史和 IGM 的热状态？
• RQ4 以主导微弱AGN为特征的情景是否与观测到的 Lyman-α 光学深度分布和平均自由程相一致？

主要发现

• 带有 17% AGN 贡献的 QSO 辅助模型要比仅星系模型的辐射发射率因子低 1.8 才能匹配 Lyman-α 透射率，并与观测到的 τ_eff 分布很好地一致。
• 仅 QSO 模型无法再现观测到的 Lyman-α 光学深度分布，且除非 He II 逃逸极低，否则会导致在 z ≲ 5 时 He II 再电离过早且 IGM 温度偏高。
• 适度的微弱AGN贡献（如 QSO 辅助情景）与 Lyman-α 森林数据一致，而微弱AGN主导的再电离难以与观测相协调。
• 在 QSO 辅助与 QSO-only 模型中，电离辐射发射率的历史和 He II 电离与星系-only 模型存在显著差异，QSO-only 情况导致再电离推迟且 HeIII 分数更高。
• 总体而言，微弱AGN在再电离中可发挥非忽略的作用，但并非主导，与 Lyman-α 约束一致。

Figure 2: The spatial distribution of galaxies (blue) and AGN (red) in our QSO-assisted model at redshifts $z=6.10$ (left panel) and $z=6.05$ (right panel). The size of the dots is a measure of the ionizing emissivity of the sources. Both panels show the same slice of the simulation volume. The thic