The origin of the supermassive black holes (SMBHs) residing in the centres of most galaxies remains a mystery. The Event Horizon Telescope has provided direct imaging of the SMBH Sagittarius A* (Sgr A*) at the Milky Way’s centre, indicating that it probably spins rapidly with its spin axis substantially misaligned relative to the Galactic plane’s angular momentum. Through investigating various SMBH growth models, here we show that the inferred spin properties of Sgr A* provide evidence of a potential past SMBH merger. Inspired by the merger between the Milky Way and the Gaia-Enceladus progenitor, which had a 4:1 mass ratio as inferred from Gaia data, we have discovered that a 4:1 major merger of a SMBH with a binary angular momentum inclination angle of 145–180° with respect to the line of sight can successfully replicate the measured spin properties of Sgr A*. This possible merger event in our Galaxy’s history provides potential observational support for the theory of hierarchical black hole mergers in the formation and growth of SMBHs. The inferred merger rate, consistent with theoretical predictions, suggests a promising detection rate of SMBH mergers for the space-borne gravitational wave detectors expected to operate in the 2030s.

大多数星系中心的超大质量黑洞（SMBH）的起源仍然是个谜。事件视界望远镜提供了位于银河系中心的 SMBH 人马座 A* (Sgr A*) 的直接成像，表明它可能快速旋转，其自转轴相对于银河平面的角动量基本上未对准。通过研究各种 SMBH 增长模型，我们在此表明​​，Sgr A* 的推断自旋特性提供了过去 SMBH 潜在合并的证据。受银河系与盖亚-土卫二前身星系合并的启发，根据盖亚数据推断，盖亚-土卫二的质量比为 4:1，我们发现，SMBH 的一次 4:1 重大合并，其二元角动量倾角为相对于视线 145–180° 可以成功复制测量到的 Sgr A* 自旋特性。我们银河系历史上这一可能的合并事件为超大质量黑洞形成和增长过程中的分层黑洞合并理论提供了潜在的观测支持。推断的合并率与理论预测一致，表明预计在 2030 年代运行的星载引力波探测器对SMBH合并的检测率很有希望。