  The origin of supermassive black holes (SMBHs) residing in the centers of
most galaxies remains a mystery. The Event Horizon Telescope (EHT) provided
direct imaging of the SMBH Sgr A* at the Milky Way's center, indicating it
likely spins rapidly with its spin axis significantly 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 past SMBH merger. Inspired by the merger between the Milky Way
and Gaia-Enceladus, which has a 4:1 mass ratio as inferred from Gaia data, we
have discovered that a 4:1 major merger of SMBH with a binary angular momentum
inclination angle of 145-180 degrees with respect to the line of sight (LOS),
can successfully replicate the measured spin properties of Sgr A*. This merger
event in our galaxy provides potential observational support for the theory of
hierarchical BH 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 space-borne gravitational wave detectors
expected to operate in 2030s.
