Evidence of a Past Merger of the Galactic Center Black Hole
arxiv(2024)
摘要
The origin of supermassive black holes (SMBHs) residing in the centers of
most galaxies remains a mystery. Various growth models, such as accretion and
hierarchical mergers, have been proposed to explain the existence and
cosmological evolution of these SMBHs, but no direct observational evidence is
available to test these models. The Event Horizon Telescope (EHT) offered
direct imaging of nearby SMBHs, in particular, the one at the center of the
Milky Way Galaxy named Sgr A*. Measurements suggest that the Sgr A* BH spins
rapidly with significant spin axis misalignment relative to the angular
momentum of the Galactic plane. Through investigating various SMBH growth
models, here we show that the spin properties of Sgr A* provides strong
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 15-45 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 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.
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