Recent observations have found a large number of supermassive black holes already in place in the first few hundred million years after the Big Bang, many of which seem to be overmassive relative to their host galaxy stellar mass when compared with local relation^{1,2,3,4,5,6,7,8,9}. Several different models have been proposed to explain these findings, ranging from heavy seeds to light seeds experiencing bursts of high accretion rate^{10,11,12,13,14,15,16}. Yet, current datasets are unable to differentiate between these various scenarios. Here we report the detection, from the JADES survey, of broad Hα emission in a galaxy at z = 6.68, which traces a black hole with a mass of about 4 × 10⁸M_⊙ and accreting at a rate of only 0.02 times the Eddington limit. The black hole to host galaxy stellar mass ratio is about 0.4—that is, about 1,000 times above the local relation—whereas the system is closer to the local relations in terms of dynamical mass and velocity dispersion of the host galaxy. This object is most likely an indication of a much larger population of dormant black holes around the epoch of reionization. Its properties are consistent with scenarios in which short bursts of super-Eddington accretion have resulted in black hole overgrowth and massive gas expulsion from the accretion disk; in between bursts, black holes spend most of their life in a dormant state.

最近的观测发现，在大爆炸后的最初几亿年里已经存在了大量超大质量黑洞，与本地关系^{1,2,3,4,5,6,7,8,9} 相比，其中许多黑洞相对于其宿主星系的恒星质量似乎质量过大。已经提出了几种不同的模型来解释这些发现，从重种子到经历高吸积率爆发的轻种子^{10,11,12,13,14,15,16}。然而，当前的数据集无法区分这些不同的场景。在这里，我们报告了 JADES 巡天在 z = 6.68 的星系中检测到的宽 Hα 发射，它追踪了一个质量约为 4 × 10⁸M_⊙ 的黑洞，吸积速率仅为爱丁顿极限的 0.02 倍。黑洞与宿主星系恒星的质量比约为 0.4，即大约是本地关系的 1,000 倍，而该系统在宿主星系的动力学质量和速度色散方面更接近本地关系。这个天体很可能表明在再电离时期周围有大量休眠的黑洞。它的特性与超级爱丁顿吸积的短爆发导致黑洞过度生长和大量气体从吸积盘中排出的情况一致;在两次爆发之间，黑洞一生中的大部分时间都处于休眠状态。