Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK.
Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK.
Nature. 2018 Jan 10;553(7687):178-181. doi: 10.1038/nature24631.
The earliest galaxies are thought to have emerged during the first billion years of cosmic history, initiating the ionization of the neutral hydrogen that pervaded the Universe at this time. Studying this 'epoch of reionization' involves looking for the spectral signatures of ancient galaxies that are, owing to the expansion of the Universe, now very distant from Earth and therefore exhibit large redshifts. However, finding these spectral fingerprints is challenging. One spectral characteristic of ancient and distant galaxies is strong hydrogen-emission lines (known as Lyman-α lines), but the neutral intergalactic medium that was present early in the epoch of reionization scatters such Lyman-α photons. Another potential spectral identifier is the line at wavelength 157.4 micrometres of the singly ionized state of carbon (the [C ii] λ = 157.74 μm line), which signifies cooling gas and is expected to have been bright in the early Universe. However, so far Lyman-α-emitting galaxies from the epoch of reionization have demonstrated much fainter [C ii] luminosities than would be expected from local scaling relations, and searches for the [C ii] line in sources without Lyman-α emission but with photometric redshifts greater than 6 (corresponding to the first billion years of the Universe) have been unsuccessful. Here we identify [C ii] λ = 157.74 μm emission from two sources that we selected as high-redshift candidates on the basis of near-infrared photometry; we confirm that these sources are two galaxies at redshifts of z = 6.8540 ± 0.0003 and z = 6.8076 ± 0.0002. Notably, the luminosity of the [C ii] line from these galaxies is higher than that found previously in star-forming galaxies with redshifts greater than 6.5. The luminous and extended [C ii] lines reveal clear velocity gradients that, if interpreted as rotation, would indicate that these galaxies have similar dynamic properties to the turbulent yet rotation-dominated disks that have been observed in Hα-emitting galaxies two billion years later, at 'cosmic noon'.
最早的星系被认为是在宇宙历史的头十亿年中出现的，它们引发了当时弥漫在宇宙中的中性氢的电离。研究这个“再电离时代”涉及到寻找由于宇宙膨胀而现在离地球非常遥远的古老星系的光谱特征，因此它们表现出很大的红移。然而，找到这些光谱指纹是具有挑战性的。古老而遥远星系的一个光谱特征是强烈的氢发射线（称为 Lyman-α 线），但在再电离时代早期存在的中性星系际介质会散射这种 Lyman-α 光子。另一个潜在的光谱标识符是波长为 157.4 微米的单电离态碳的线（[C ii] λ=157.74μm 线），它表示冷却气体，预计在早期宇宙中会很亮。然而，到目前为止，再电离时代的 Lyman-α 发射星系的 [C ii] 亮度比根据局部标度关系所预期的要暗得多，并且对没有 Lyman-α 发射但具有大于 6 的光度红移（对应于宇宙的头十亿年）的源的 [C ii] 线的搜索都没有成功。在这里，我们在基于近红外测光选择的高红移候选源中识别出了 [C ii] λ=157.74μm 发射；我们确认这些源是两个红移为 z=6.8540±0.0003 和 z=6.8076±0.0002 的星系。值得注意的是，这些星系的 [C ii] 线的光度高于以前在红移大于 6.5 的恒星形成星系中发现的光度。明亮且扩展的 [C ii] 线揭示了清晰的速度梯度，如果解释为旋转，那么这表明这些星系具有与在 Hα 发射星系中观察到的、两亿年后处于“宇宙正午”的、具有湍流但以旋转为主的盘相似的动力学特性。
