[论文解读] UNCOVERing the contribution of black holes to reionization in the JWST era

Pratika Dayal, Marta Volonteri+14 more|arXiv (Cornell University)|Jan 20, 2024

Galaxies: Formation, Evolution, Phenomena被引用 10

一句话总结

该研究使用 Delphi 半解析模型，对 JWST/ALMA 数据进行标定，量化再电离期间的电离光子预算，显示低质量星系在早期占主导地位，而 AGN 仅在结束阶段变得重要。

ABSTRACT

With its sensitivity in the rest-frame optical, the James Webb Space Telescope (JWST) has uncovered active galactic nuclei (AGN), comprising both intrinsically faint and heavily reddened sources, well into the first billion years of the Universe, at $z \sim 4-11$. In this work, we revisit the AGN contribution to reionization given the high number densities associated with these objects. We use the DELPHI semi-analytic model, base-lined against the latest high-redshift datasets from the JWST and the Atacama Large millimetre Array (ALMA) to model early star forming galaxies and AGN. We calculate the escape fractions of ionizing radiation from both star formation and AGN and include the impact of reionization feeback in suppressing the baryonic content of low-mass galaxies in ionized regions. This model is validated against the key observables for star forming galaxy, AGN and reionization. In our {\it fiducial} model, reionization reaches its mid-point at $z \sim 6.9$ and ends by $z \sim 5.9$. Low stellar mass ($M_*\leq 10^9M_\odot$) star forming galaxies are found to be the key drivers of the reionization process, providing about $77\%$ of the total photon budget. Despite their high numbers, high accretion rates and higher escape fractions compared to star forming galaxies at $z \sim 5$, AGN only provide about $23\%$ of the total reionization budget which is dominated by black holes in high stellar mass systems (with $M_* \geq 10^9M_\odot$). This is because AGN number densities become relevant only at $z \leq 7$ - as a result, AGN contribute as much as galaxies as late as $z \sim 6.2$, when reionization is already in its end stages. Finally, we find that even contrasting models of the AGN ionizing photon escape fraction (increasing or decreasing with stellar mass) do not qualitatively change our results.

研究动机与目标

• 评估星形成星系与 AGN 在宇宙再电离中的相对作用。
• 将半解析模型（Delphi）标定为来自 JWST/ALMA 的高-z 星系与 AGN 观测。
• 计算电离光子逃逸分数与再电离反馈效应。
• 在紫外光度函数和再电离观测量（τ_es、Q_II）上对模型进行验证。
• 探讨不同的 AGN 逃逸分数处方如何影响再电离史。

提出的方法

• 采用 Delphi 半解析模型，跟踪从 z~40 到 z=4.5 的暗晕、气体、恒星、金属、尘埃和黑洞。
• 以重种子黑洞（10^3–10^5 M_sun）种子，并通过吸积和合并实现增长，放射效率为（ε_r=0.1）。
• 实现一个随红移演化的高效 BH 吸积的临界质量并带有散射（0.5 dex）。
• 采用对尘埃有衰减的星逃逸分数 f_esc^sf，与 UV 光谱斜率 β 以及金属/尘埃预算相关联；通过对 BH 的尘埃衰减光学深度 τ_bh 来模型 AGN 逃逸分数 f_esc^bh。
• 在体积中以电离分数 Q_II 对发射率进行加权，在质量为 50 km/s 以下的晕中采用最大 UV 背景抑制情形来包含再电离反馈。
• 通过将星形成和 AGN 的贡献相加并各自乘以 Q_II 和 Q_I（中性区域）的权重，计算电离发射率 dot{n}_ion。

Figure 1 : The rest-frame UV LF at $z\sim 5$ , $7$ and $10$ , as marked. In each panel, the long-dashed, dot-dashed and solid lines show the dust obscured UV LF for AGN, star formation and the “total” luminosity, as marked. In all panels, points show observational results as marked: at $z\sim 5$ (pa

实验结果

研究问题

• RQ1 星形成星系与 AGN 在再电离中的电离光子预算中各自的相对贡献如何？
• RQ2 尘埃衰减与逃逸分数如何影响在 redshift(z~5–10) 与星系质量下星际与黑洞的电离光子逃逸？
• RQ3 一个经过 JWST 标定的 Delphi 模型是否能在 z~5–10 范围内再现观测到的 UV 光度函数和再电离观测量（τ_es, Q_II）？
• RQ4 再电离反馈如何改变低质量晕中的重 baryonic 内容以及随后的恒星形成/黑洞增长？
• RQ5 不同的 AGN 逃逸分数处方对再电离的时序与持续时间有何影响？

主要发现

• 低质量的恒星形成星系（M_* < 10^9 M_sun）驱动了再电离的大部分，在 fiducial 模型中约贡献 total 电离光子预算的 77%。
• AGN 贡献约 23% 的总再电离预算，主要由高质量星系（M_* ≳ 10^9 M_sun）中的黑洞在 z ≤ 7 时主导。
• AGNs 的数密度仅在 z ≲ 7 时显著，AGN 对再电离的贡献在再电离末期（z ~ 5.9–6.2）达到峰值。
• 即使采用不同的 AGN f_esc^bh 处方（随质量变动或与质量无关），结果的定性结论仍然成立：低质量星系中的恒星引领再电离；AGN 主要在后期阶段贡献，而非提前主导预算。
• 一个最大 f_esc^bh = 1 的情景在 z ≲ 6 时会过度产生发射率，与观测不符，因此不被支持。
• fiducial 模型给出再电离中点在 z ≈ 6.9，完成在 z ≈ 5.9，τ_es 与 Planck 值一致。

Figure 2 : As a function of the stellar mass, we show the intrinsic production rate of ionizing photons ( left panel ) and their escape fractions ( right panel ) at $z\sim 5-10$ , as marked. In each panel, the solid and dashed lines show the results for star formation and black holes, respectively.