Evidence of a Past Merger of the Galactic Center Black Hole (2403.06416v4)

Abstract: 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.

• The paper demonstrates that EHT observations reveal Sgr A*’s rapid, misaligned spin indicative of a past major merger.
• The paper employs simulations using the NRSur7dq4EmriRemnant model to replicate observed spin dynamics and support merger interpretations.
• The paper links its findings to cosmological implications by predicting observable SMBH merger rates for future gravitational wave detectors like LISA.

Evidence of a Past Merger of the Galactic Center Black Hole

The paper under review provides a comprehensive analysis of the spin properties of the supermassive black hole (SMBH) Sgr A* located at the center of the Milky Way. The investigation leverages the observational data from the Event Horizon Telescope (EHT), which suggests that Sgr A* exhibits rapid rotation with its spin axis significantly misaligned relative to the angular momentum of the galaxy's plane. The authors propose a past merger event as a plausible explanation for these spin characteristics, challenging traditional SMBH growth models based solely on accretion.

Spin Constraints and Their Implications

The inference of a rapidly spinning Sgr A* with a misaligned spin axis presents challenges to established accretion-based SMBH growth models. Both coherent and chaotic accretion models face limitations, as coherent models are inconsistent with a significant spin misalignment, while chaotic models are unlikely to result in high spin magnitudes due to their episodic nature. Therefore, these models are insufficient to simultaneously explain the high spin and misalignment observed for Sgr A*.

Merger Models as a Solution

The authors explore the possibility of a past hierarchical merger to account for Sgr A*'s spin properties. They simulate the evolution of SMBH growth through accretion and merger models starting from different SMBH seeds, considering both coherent and chaotic accretion processes. Furthermore, they utilize the surrogate BH merger model NRSur7dq4EmriRemnant to predict post-merger spin characteristics.

A critical aspect of the model is the proposed 4:1 major merger likelihood between two galaxies, correlating with the Milky Way's merger history with Gaia-Enceladus about 10 billion years ago. The paper finds that a major merger with specific binary angular momentum inclination angles can reproduce the observed spin properties of Sgr A*. This merger interpretation is supported by the constraints from the EHT observations, which suggest specific pre-merger spin conditions.

Cosmological Implications and Future Prospects

The paper ties these findings into broader cosmological simulations, which predict a low galaxy merger rate. However, the merging history reflected by Sgr A* supports the theory that SMBH mergers significantly contribute to the Sgr A* properties observed. The estimated major merger rate density from simulations is consistent with theoretical predictions, suggesting observable rates of SMBH mergers for upcoming space-borne gravitational wave detectors, such as LISA, Taiji, and TianQin.

These detectors, expected to operate in the 2030s, present opportunities for further validation of this merger hypothesis through direct observations of SMBH binaries. Thus, the paper not only elucidates a historical perspective on Sgr A*'s growth but also anticipates a promising future for gravitational wave astronomy in confirming SMBH merger events.

Conclusion

This research presents a robust argument for the role of past mergers in shaping the spin dynamics of Sgr A*, offering a cohesive explanation that aligns with both empirical observations and theoretical models. The implications extend beyond local observations, placing this paper at the convergence of observational astrophysics and theoretical cosmology, with promising prospects for future gravitational wave detector missions.