The Predicament of Absorption-dominated Reionization: Increased Demands on Ionizing Sources
The Predicament of Absorption-dominated Reionization: Increased Demands on Ionizing Sources
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DOI：
10.3847/2041-8213/ac1ffb
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发表时间：
2021-05
期刊：
影响因子：
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通讯作者：
F. Davies;S. Bosman;S. Furlanetto;G. Becker;Anson D’Aloisio
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作者：
F. Davies;S. Bosman;S. Furlanetto;G. Becker;Anson D’Aloisio
当电离光子到达宇宙的每一个角落时，再电离时代结束。再电离通常被认为主要受星系产生电离光子的速率限制，但贝克尔等人最近测量到z = 6时电离光子的平均自由程为0.75−0.45+0.65 Mpc，这一结果令人惊讶，这表明星系际介质中剩余的中性氢的吸收可能比以前预期的要大得多。在这里，我们证明了这个短的平均自由程和Lyα森林中同时代的暗像素分数之间的一致性，需要在再电离结束时每个重子累积输出6.1−2.4+11个电离光子，远高于通常所需的1-3。这意味着电离光子预算比以前的估计有了显著的增加，大大加剧了以后对星系电离输出测量的紧张局势。在我们的模型中，将这个约束转化为星系的瞬时电离产物，我们发现在z = 6时，log 10 fesc π介子/（erg/Hz）−1=25.02−0.21+0.45。即使对早期恒星群体的电离产生效率有乐观的假设，并假设星系光度函数扩展到极暗的源（M UV ≤ − 11），完全再电离需要电离光子的逃逸分数超过20%。这比在任何较低红移的星系群中观察到的要大得多，这需要在宇宙时间的第一个十亿年后星系性质的快速演化。在现有的氢中性分数和平均自由程的观测限制下，这种紧张局势不能完全缓解。
The reionization epoch concludes when ionizing photons reach every corner of the universe. Reionization has generally been assumed to be limited primarily by the rate at which galaxies produce ionizing photons, but the recent measurement of a surprisingly short ionizing photon mean free path of 0.75−0.45+0.65 proper Mpc at z = 6 by Becker et al. suggests that absorption by residual neutral hydrogen in the otherwise ionized intergalactic medium may play a much larger role than previously expected. Here we show that consistency between this short mean free path and the coeval dark pixel fraction in the Lyα forest requires a cumulative output of 6.1−2.4+11 ionizing photons per baryon by reionization’s end, well above the typically required ∼1–3. This represents a dramatic increase in the ionizing photon budget over previous estimates, greatly exacerbating the tension with measurements of the ionizing output from galaxies at later times. Translating this constraint into the instantaneous ionizing production from galaxies in our model, we find log10fescξion/(erg/Hz)−1=25.02−0.21+0.45 at z ∼ 6. Even with optimistic assumptions about the ionizing production efficiency of early stellar populations, and assuming the galaxy luminosity function extends to extremely faint sources (M UV ≤ − 11), complete reionization requires the escape fraction of ionizing photons to exceed 20% across the galaxy population. This is far larger than observed in any galaxy population at lower redshifts, requiring rapid evolution in galaxy properties after the first billion years of cosmic time. This tension cannot be completely relieved within existing observational constraints on the hydrogen neutral fraction and mean free path.