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Evidence of a past merger of the Galactic Centre black hole

Nature Astronomy (2024)Cite this article

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Abstract

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.

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Fig. 1: Examples of mass and spin evolution of coherent accretion models and chaotic accretion models from three different SMBH seeds.
Fig. 2: Kernel density estimates of the final BH spin and orientation for Sgr A*-like SMBHs accreted from direct-collapse SMBH seeds.
Fig. 3: Schematics of the SMBH merger model.
Fig. 4: Kernel density estimates of the final BH spin and orientation from the 4:1 merger models.
Fig. 5: SMBH binary formation and merger rates (R) at different redshifts for different primary BH masses and mass ratios.

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Data availability

Data are available via figshare at https://figshare.com/articles/dataset/Sgr_A_data/26112379 (ref. 109).

Code availability

The code and data-processing script are available at https://github.com/YihanWangAstro/Sgr-A-PubCode.

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Acknowledgements

Y.W. and B.Z. acknowledge support from NASA (Grant No. 80NSSC23M0104) and the Nevada Center for Astrophysics. Y.W. acknowledges useful discussions with T. Bogdanovic regarding the recoil velocity of major mergers, with D. N. C. Lin on subsequent accretions following the merger, as well as with B. McKernan and D. Stern concerning coherent accretions.

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Authors and Affiliations

  1. Nevada Center for Astrophysics, University of Nevada, Las Vegas, NV, USA

    Yihan Wang & Bing Zhang

  2. Department of Physics and Astronomy, University of Nevada, Las Vegas, NV, USA

    Yihan Wang & Bing Zhang

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  1. Yihan Wang
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Contributions

B.Z. proposed the idea of this paper. Y.W. developed the theoretical models. Y.W. and B.Z. analysed the results and discussed the theoretical models. Both authors contributed to the analysis or interpretation of the data and to the final version of the manuscript.

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Correspondence to Yihan Wang or Bing Zhang.

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Extended data

Extended Data Fig. 1 SMBH seeds mass distribution from three different formation channels.

This figure presents the mass distribution of SMBH seeds derived from three distinct formation channels. The blue distribution represents SMBH seeds formed from population-III stars. The orange distribution illustrates the SMBH seeds resulting from cluster runaway collisions. The green distribution shows the mass distribution from direct collapse of clouds and dark matter.

Extended Data Fig. 2 Distribution of the accretion disk misalignment angle for different values of k in Mises function.

k = 0 indicates isotropically distributed accretion model, while large value of k (~ > 30) is asymptotic to coherent accretion model.

Extended Data Fig. 3 Kernel density estimates of the final BH spin and orientation for Sgr A*-like SMBHs accreted from Pop-III SMBH seeds.

The blue contours represent the chaotic accretion models with isotropic disk orientation, while the yellow contours represent the coherent accretion models. The left panels show the jet-free case, the middle panels show the weak BZ jet case, and the right panels show the strong BZ jet case. From top to bottom, the effective accretion rate increases. The red blocks represent regions disfavored by EHT constraints, whereas the green blocks indicate the ‘best-bet’ regions of parameter space that perform well and explain nearly all observed data, excluding polarization. The region marked with slashes highlights the ‘best-bet’ area, taking into account the polarization constraints. The absence of contours indicates failures to accrete to Sgr A* mass within the Hubble time.

Extended Data Fig. 4 Kernel density estimates of the final BH spin and orientation for Sgr A*-like SMBHs accreted from star cluster runaway SMBH seeds.

The blue contours represent the chaotic accretion models with isotropic disk orientation, while the yellow contours represent the coherent accretion models. The left panels show the jet-free case, the middle panels show the weak BZ jet case, and the right panels show the strong BZ jet case. From top to bottom, the effective accretion rate increases. The red blocks represent regions disfavored by EHT constraints, whereas the green blocks indicate the ‘best-bet’ regions of parameter space that perform well and explain nearly all observed data, excluding polarization. The region marked with slashes highlights the ‘best-bet’ area, taking into account the polarization constraints.

Extended Data Fig. 5 Final spin magnitude and spin-orbital misalignment angle for 4:1 SMBH binary major mergers, differentiated by pre-merger spin vectors a1 and a2.

The color coding corresponds to different initial magnitudes of a1. The left panel displays scenarios where the secondary SMBH has a negligible initial spin, whereas the right panel represents cases with a nearly maximally spinning secondary SMBH. The pre-merger spins a1 and a2 are assumed to be isotropically distributed.

Extended Data Fig. 6 Kernel density estimates of the final BH spin and orientation from the 8:1 merger models.

a1 indicates the spin magnitudes of the primary and secondary SMBHs before the merger, respectively. The value of ϕ represents different binary SMBH orientations with respect to the LOS. The orientations of a1 are isotropically distributed to encompass both the accretion-only and merger progenitor cases. The final spin distributions show very weak dependency on a2.

Extended Data Fig. 7 Kernel density estimates of the final BH spin and orientation from the 16:1 merger models.

a1 indicate the spin magnitudes of the primary and secondary SMBHs before the merger, respectively. The value of ϕ represents different binary SMBH orientations with respect to the LOS. The orientations of a1 are isotropically distributed to encompass both the accretion-only and merger progenitor cases. The final spin distributions show very weak dependency on a2.

Extended Data Fig. 8 Velocity Distribution and Settling Time Post-4:1 Merger.

The left panel displays the distribution of the recoil kick velocities immediately following the 4:1 merger. The right panel depicts the required settling time for Sgr A* to return to the observed Brownian motion level at the Galactic Center.

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Wang, Y., Zhang, B. Evidence of a past merger of the Galactic Centre black hole. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02358-w

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