Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada.
Department of Astronomy, Yale University, New Haven, CT, USA.
Nature. 2018 Apr;556(7702):469-472. doi: 10.1038/s41586-018-0025-2. Epub 2018 Apr 25.
Massive galaxy clusters have been found that date to times as early as three billion years after the Big Bang, containing stars that formed at even earlier epochs. The high-redshift progenitors of these galaxy clusters-termed 'protoclusters'-can be identified in cosmological simulations that have the highest overdensities (greater-than-average densities) of dark matter. Protoclusters are expected to contain extremely massive galaxies that can be observed as luminous starbursts . However, recent detections of possible protoclusters hosting such starbursts do not support the kind of rapid cluster-core formation expected from simulations : the structures observed contain only a handful of starbursting galaxies spread throughout a broad region, with poor evidence for eventual collapse into a protocluster. Here we report observations of carbon monoxide and ionized carbon emission from the source SPT2349-56. We find that this source consists of at least 14 gas-rich galaxies, all lying at redshifts of 4.31. We demonstrate that each of these galaxies is forming stars between 50 and 1,000 times more quickly than our own Milky Way, and that all are located within a projected region that is only around 130 kiloparsecs in diameter. This galaxy surface density is more than ten times the average blank-field value (integrated over all redshifts), and more than 1,000 times the average field volume density. The velocity dispersion (approximately 410 kilometres per second) of these galaxies and the enormous gas and star-formation densities suggest that this system represents the core of a cluster of galaxies that was already at an advanced stage of formation when the Universe was only 1.4 billion years old. A comparison with other known protoclusters at high redshifts shows that SPT2349-56 could be building one of the most massive structures in the Universe today.
已经发现了大量的星系团，它们的形成时间可以追溯到大爆炸之后的 30 亿年，其中包含了更早时期形成的恒星。这些星系团的高红移先驱体——称为“原星系团”——可以在宇宙学模拟中被识别出来，这些模拟具有最高的暗物质超密度（高于平均密度）。原星系团预计包含极其巨大的星系，可以观察到它们作为发光的星暴星系团。然而，最近对可能包含这种星暴的原星系团的探测结果并不支持模拟所预期的那种快速星系团核心形成：观察到的结构只包含少数几个散布在广阔区域中的星暴星系，几乎没有证据表明最终会坍缩成原星系团。在这里，我们报告了对源 SPT2349-56 的一氧化碳和离子化碳发射的观测结果。我们发现，这个源至少包含 14 个富气体星系，它们都位于红移为 4.31 的位置。我们证明，这些星系中的每一个都在形成恒星，速度比我们的银河系快 50 到 1000 倍，而且它们都位于一个只有大约 130 千秒差距直径的投影区域内。这个星系的表面密度比平均空白场值（在所有红移上积分）高出 10 倍以上，比平均场体积密度高出 1000 倍以上。这些星系的速度弥散度（约为 410 公里/秒）和巨大的气体和恒星形成密度表明，这个系统代表了一个星系团的核心，当宇宙只有 14 亿岁时，这个星系团已经处于形成的高级阶段。与其他已知的高红移原星系团的比较表明，SPT2349-56 可能正在构建当今宇宙中最庞大的结构之一。
