近日，法国巴黎天体物理研究所的Hakim Atek及其研究团队取得一项新进展。经过不懈努力，他们发现大多数使宇宙再电离的光子来自矮星系。相关研究成果已于2024年2月28日在国际权威学术期刊《自然》上发表。

该研究团队报道了对8个超微弱星系在再电离时期的深入分析，这些星系位于一个极小的视场内，其绝对星等介于M_UV≈-17等和-15等之间（低至0.005L*）。研究人员发现，在宇宙诞生后的前10亿年里，这些暗星系产生的电离光子数量达到了log[ξ_ion(Hzerg-1)]= 25.80±0.14，这比通常假设的值高出4倍。如果这一小片区域代表了微弱星系在宇宙中的广泛分布，那么电离光子的产生速率将足以超过宇宙再电离所需的速度，即使仅有5%的电离光子成功逃逸至星际空间。

据悉，宇宙再电离是宇宙大爆炸后从中性氢转变为电离等离子体的关键过程，大约发生在距今6亿至8亿年前。关于驱动这一过程的源头，学界一直存在争议。部分模型认为，类星体因其高电离发射率和逃逸分数（f_esc）在推动宇宙再电离中扮演了重要角色。另一些人则提出，明亮星系的高f_esc值能产生充足的电离辐射以驱动这一过程。还有研究显示，当结合恒星质量依赖的电离效率和f_esc模型时，微弱星系的数密度能够有效地控制宇宙再电离。然而，截至目前，针对低质量星系的光谱研究尚不全面，这主要是因为它们的光度极为微弱。

附：英文原文

Title: Most of the photons that reionized the Universe came from dwarf galaxies

Author: Atek, Hakim, Labb, Ivo, Furtak, Lukas J., Chemerynska, Iryna, Fujimoto, Seiji, Setton, David J., Miller, Tim B., Oesch, Pascal, Bezanson, Rachel, Price, Sedona H., Dayal, Pratika, Zitrin, Adi, Kokorev, Vasily, Weaver, John R., Brammer, Gabriel, Dokkum, Pieter van, Williams, Christina C., Cutler, Sam E., Feldmann, Robert, Fudamoto, Yoshinobu, Greene, Jenny E., Leja, Joel, Maseda, Michael V., Muzzin, Adam, Pan, Richard, Papovich, Casey, Nelson, Erica J., Nanayakkara, Themiya, Stark, Daniel P., Stefanon, Mauro, Suess, Katherine A., Wang, Bingjie, Whitaker, Katherine E.

Issue&Volume: 2024-02-28

Abstract: The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600–800Myr after the Big Bang, has been a matter of debate. Some models suggest that high ionizing emissivity and escape fractions (f_esc) from quasars support their role in driving cosmic reionization. Others propose that the high f_esc values from bright galaxies generate sufficient ionizing radiation to drive this process. Finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and f_esc, can effectively dominate cosmic reionization. However, so far, comprehensive spectroscopic studies of low-mass galaxies have not been done because of their extreme faintness. Here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between M_UV≈-17mag and -15mag (down to 0.005L*. We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ξ_ion(Hzerg^-1)]= 25.80±0.14, a factor of 4 higher than commonly assumed values. If this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%.

DOI: 10.1038/s41586-024-07043-6

Source: https://www.nature.com/articles/s41586-024-07043-6