Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Roma, Rome, Italy.
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA.
Nature. 2018 Jun;558(7710):406-409. doi: 10.1038/s41586-018-0204-1. Epub 2018 Jun 20.
It has been known for decades that the observed number of baryons in the local Universe falls about 30-40 per cent short of the total number of baryons predicted by Big Bang nucleosynthesis, as inferred from density fluctuations of the cosmic microwave background and seen during the first 2-3 billion years of the Universe in the so-called 'Lyman α forest' (a dense series of intervening H I Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas-hydrogen-is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The O VII systems lie in regions characterized by large (four times larger than average ) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium. We conclude that the missing baryons have been found.
几十年来，人们已经知道，在局部宇宙中观测到的重子数量比大爆炸核合成推断出的总重子数量少了约 30-40%，这可以从宇宙微波背景的密度波动中看出，也可以在宇宙的前 2-30 亿年中看到，也就是所谓的“莱曼阿尔法森林”（在背景类星体的光学光谱中，密集的一系列中性氢莱曼阿尔法吸收线）。这个悖论的一个理论解决方案是将缺失的重子定位在星系之间的热而稀疏的丝状气体中，这种气体被称为暖热星系际介质。然而，在那里很难探测到它们，因为到目前为止，这种气体中最大的成分——氢——大部分是电离的，因此在远紫外光谱中几乎不可见，而远紫外光谱的信噪比通常很低。事实上，尽管进行了大量的观测努力，但到目前为止，只有少数几个边缘的探测结果被报道。在这里，我们报告了在一个红移高于 0.4 的类星体的高信噪比 X 射线光谱中观测到两个高度电离氧（O VII）吸收体的结果。这些吸收体在两年的时间尺度内没有变化，也没有相关的冷吸收，这使得假设它们来自类星体的固有外流或宿主星系的星际介质不太可能。O VII 系统位于星系过密度较大的区域（比平均水平大四倍），其数量（达到我们数据的灵敏度阈值）与长期以来寻求的暖热星系际介质的数值模拟预测相符。我们得出结论，失踪的重子已经被发现了。
