SIMBAD references

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.