Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
Nature. 2012 Sep 20;489(7416):406-8. doi: 10.1038/nature11446.
Re-ionization of the intergalactic medium occurred in the early Universe at redshift z ≈ 6-11, following the formation of the first generation of stars. Those young galaxies (where the bulk of stars formed) at a cosmic age of less than about 500 million years (z ≲ 10) remain largely unexplored because they are at or beyond the sensitivity limits of existing large telescopes. Understanding the properties of these galaxies is critical to identifying the source of the radiation that re-ionized the intergalactic medium. Gravitational lensing by galaxy clusters allows the detection of high-redshift galaxies fainter than what otherwise could be found in the deepest images of the sky. Here we report multiband observations of the cluster MACS J1149+2223 that have revealed (with high probability) a gravitationally magnified galaxy from the early Universe, at a redshift of z = 9.6 ± 0.2 (that is, a cosmic age of 490 ± 15 million years, or 3.6 per cent of the age of the Universe). We estimate that it formed less than 200 million years after the Big Bang (at the 95 per cent confidence level), implying a formation redshift of ≲14. Given the small sky area that our observations cover, faint galaxies seem to be abundant at such a young cosmic age, suggesting that they may be the dominant source for the early re-ionization of the intergalactic medium.
太初宇宙中，星系际介质在红移 z ≈ 6-11 时发生再电离，此时第一代恒星形成。在宇宙年龄小于约 5 亿年（z≲10）时形成的年轻星系（大部分恒星在此形成）在很大程度上仍未被探索，因为它们处于或超出了现有大型望远镜的灵敏度极限。了解这些星系的性质对于确定重新电离星系际介质的辐射源至关重要。星系团的引力透镜作用可以探测到红移高于天空最深图像所能探测到的高红移星系。在这里，我们报告了对 MACS J1149+2223 星系团的多波段观测结果，这些观测结果（极有可能）揭示了一个来自早期宇宙的处于红移 z = 9.6±0.2 的引力放大星系（即宇宙年龄为 490±15 百万年，或宇宙年龄的 3.6%）。我们估计它形成于大爆炸后不到 2 亿年（置信度为 95%），这意味着形成红移 ≲14。鉴于我们的观测所覆盖的天空面积较小，在如此年轻的宇宙年龄下，似乎存在大量的暗弱星系，这表明它们可能是星系际介质早期再电离的主要来源。
