Evidence for a Recent Supermassive Black Hole Binary in the Galactic Center from Hypervelocity Stars

Основные понятия

The observed deficit of hypervelocity stars (HVSs) with speeds above 700 km/s in the Galactic halo, along with the age and distribution of S-stars in the Galactic Center, strongly suggests the recent presence of a supermassive black hole binary (SMBHB) in the Galactic Center. This binary, likely formed after the merger of the Milky Way with the Gaia-Sausage-Enceladus dwarf galaxy, would have influenced the ejection velocities of HVSs and shaped the current stellar population near the Galactic Center.

Аннотация

• Bibliographic Information: Cao, C.Y., Liu, F.K., Li, S., Chen, X., & Wang, K. (2024). A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by the Hypervelocity Stars. arXiv preprint arXiv:2411.09278v1.

• Research Objective: This study investigates the observed velocity distribution of B-type hypervelocity stars (HVSs) in the Galactic halo, particularly the lack of HVSs with speeds exceeding 700 km/s, which contradicts existing models. The authors aim to explain this discrepancy and explore its implications for the history of the Galactic Center.

• Methodology: The researchers conducted extensive numerical scattering experiments to simulate the interactions of stars and binaries with a supermassive black hole (SMBH) and a SMBH binary (SMBHB). They considered different loss cone regimes (full and empty) to model the efficiency of stellar interactions with the central massive objects. By comparing their simulation results with the observed properties of HVSs and S-stars, they constrained the parameters of a potential past SMBHB in the Galactic Center.

• Key Findings: The study's findings suggest that the observed HVS velocity distribution, particularly the high-velocity deficit, can be best explained by the presence of an orbiting intermediate-mass black hole (IMBH) around the central SMBH, forming a SMBHB, until approximately 10 million years ago. This SMBHB, with a secondary black hole mass of about 15,000 solar masses, would have tidally disrupted stars and binaries, ejecting some as HVSs and leaving others bound to the center, forming the S-star cluster.

• Main Conclusions: The authors propose that the Milky Way's merger history, specifically with the Gaia-Sausage-Enceladus (GSE) dwarf galaxy, likely led to the formation of the SMBHB. The GSE merger, estimated to have occurred around 10 billion years ago, could have brought the IMBH into the Galactic Center, where it interacted with the SMBH. The subsequent evolution and eventual coalescence of this SMBHB would have significantly impacted the dynamics of stars in the Galactic Center, producing the observed HVS population and shaping the S-star cluster.

• Significance: This research provides compelling evidence for a recent SMBHB in the Milky Way's history, offering valuable insights into the processes of galaxy mergers, SMBH growth, and the dynamical evolution of galactic nuclei.

• Limitations and Future Research: The study acknowledges uncertainties in the observed proper motions of HVSs and the two-body relaxation timescale in the Galactic Center. Future research with more precise measurements of HVSs and improved models of stellar dynamics in galactic nuclei will further refine our understanding of the SMBHB's history and its impact on the Milky Way.

Статистика

Dozens of B-type hypervelocity stars (HVSs) moving faster than the Galactic escape speed have been discovered in the Galactic halo.
No HVS is found above 700 km/s in the halo.
About 40%-60% HVSs are expected to have velocities above 700 km/s based on previous models.
The SMBH-IMBH binary coalesced about 10 million years ago.
The secondary black hole has a mass of about 15,000 solar masses.
The SMBHB had a semimajor axis of about 160 AU and an eccentricity of about 0.4 at the lookback time of 500 Myr.
The GSE dwarf galaxy had a total stellar mass of roughly 5 × 10⁸ solar masses.
The recoil velocity from the SMBHB coalescence is about 0.5 km/s.

Цитаты

"The observed velocity distribution of the B-type HVSs and in particular the deficiency of the HVSs above 700 km/s is severely at odds with the expectations of the present models for HVSs."
"Here we show that the deficiency of the tidal disruptions of B-type binaries with a∗≲0.3 AU is due to the blocking effect of an orbiting IMBH of mass about 15, 000M⊙ with semimajor axis (aMBHB) about 90 AU at the lookback time about 100 Myr."
"The SMBH-IMBH binary formed probably after the merger of the Galaxy with the Gaia-Sausage-Enceladus (GSE) dwarf galaxy, and coalesced about 10 million years ago."

Ключевые выводы из

A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by the Hypervelocity Stars

by C.Y. Cao (PK... в arxiv.org 11-15-2024

https://arxiv.org/pdf/2411.09278.pdf

Дополнительные вопросы

How might future observations of gravitational waves from merging SMBHBs in other galaxies provide further support for this model and offer insights into the evolution of such systems?

Answer:
Future observations of gravitational waves (GW) from merging SMBHBs in other galaxies hold immense potential to revolutionize our understanding of these systems and provide compelling evidence for models like the one proposed in the paper. Here's how:

Confirmation of SMBHB Prevalence: Detecting GW signals from merging SMBHBs across the universe would directly confirm their existence and provide insights into their population statistics. This would be crucial in establishing whether SMBHB events are common outcomes of galaxy mergers, as theorized.
Mapping SMBHB Evolution: The characteristics of the GW signal, such as frequency evolution and amplitude, encode crucial information about the SMBHB's orbital parameters, masses, and spins. By analyzing these signals, we can trace the final stages of SMBHB inspiral and merger, providing a direct comparison to theoretical models like the one presented for the Milky Way.
Probing the Empty Loss Cone Regime: The paper argues for an "empty loss cone" scenario, where the SMBHB's influence disrupts the smooth flow of stars towards the center. GW observations can help confirm this by correlating the presence of SMBHBs with specific stellar dynamical features in galactic nuclei.
Constraining SMBH Growth Models: The mass ratio of merging SMBHBs, as revealed by GW signals, can provide vital clues about the hierarchical growth of SMBHs. Frequent mergers with smaller companions would favor a hierarchical growth model, while mergers with comparable mass ratios might suggest alternative formation scenarios.
Synergy with Electromagnetic Observations: Combining GW observations with electromagnetic counterparts, such as flares or changes in galactic nuclei emissions, can offer a more comprehensive picture of the merger process. This multi-messenger approach can help us understand the interplay between SMBHBs, their surrounding gas, and star formation in galactic centers.
Observatories like LISA (Laser Interferometer Space Antenna) and the proposed Einstein Telescope are specifically designed to detect GW from merging SMBHBs. These future observations promise to revolutionize our understanding of these enigmatic objects and their role in galaxy evolution.

Could alternative mechanisms, such as interactions with a dense stellar cusp or a previously unconsidered population of intermediate-mass black holes, explain the observed HVS velocity distribution without invoking a recent SMBHB?

Answer:
While the paper presents a compelling case for a recent SMBHB in the Milky Way based on the observed HVS velocity distribution, it's crucial to consider alternative explanations. Here are some possibilities:

Dense Stellar Cusp: A highly concentrated cusp of stars around the SMBH could, in principle, scatter stars to high velocities through close gravitational encounters. However, achieving the observed HVS velocities (especially the deficit above 700 km/s) would require an extremely dense and compact cusp, which might be challenging to reconcile with other observational constraints on the Milky Way's nuclear star cluster.
Unconsidered IMBH Population: The presence of a previously undetected population of IMBHs in the Galactic Center could potentially scatter stars to high velocities. However, this scenario would need to explain the specific velocity distribution of HVSs, including the high-velocity deficit. Additionally, the spatial distribution and dynamics of such an IMBH population would need to be consistent with other observations of the Galactic Center.
Non-Standard Hills Mechanism: Modifications to the standard Hills mechanism, such as interactions involving hierarchical triple stars or the presence of a massive perturber other than an IMBH, could potentially alter the expected HVS velocity distribution. Exploring these variations on the Hills mechanism through detailed numerical simulations would be necessary to assess their viability.
Observational Biases: It's essential to consider potential observational biases in the HVS sample. For instance, selection effects in the surveys used to identify HVSs could lead to an incomplete picture of the true velocity distribution. Further observations and a better understanding of these biases are crucial for robustly testing different models.
While these alternative mechanisms could contribute to the observed HVS population, they face challenges in fully explaining the specific features of the velocity distribution and other observational constraints. The SMBHB model, as presented in the paper, currently offers a more comprehensive and consistent explanation.

If the Milky Way's history of galactic mergers plays a crucial role in shaping the dynamics of its central region, what implications might this have for our understanding of galaxy evolution and the prevalence of SMBHBs in the universe?

Answer:
The idea that the Milky Way's history of galactic mergers has significantly shaped the dynamics of its central region, including the potential presence of a recent SMBHB, has profound implications for our broader understanding of galaxy evolution and the prevalence of SMBHBs in the universe:

Galaxy Mergers as SMBHB Drivers: If the Milky Way's SMBHB formation is linked to a past merger with the Gaia-Sausage-Enceladus dwarf galaxy, it strengthens the idea that galaxy mergers are key drivers of SMBHB formation and evolution. This supports the hierarchical galaxy formation model, where galaxies grow through a series of mergers, leading to the coalescence of their central SMBHs.
Prevalence of SMBHBs: The presence of a recent SMBHB in the Milky Way, a relatively typical spiral galaxy, suggests that such events might be common in the universe. This has significant implications for our understanding of SMBH growth, as SMBHB mergers are thought to be a primary mechanism for SMBHs to reach their observed masses.
Impact on Galactic Nuclei: SMBHBs can dramatically influence the dynamics and evolution of galactic nuclei. Their gravitational influence can disrupt star formation, scatter stars and gas out of the galactic center, and trigger AGN activity. Understanding the frequency and properties of SMBHBs is crucial for modeling the evolution of galactic nuclei and their role in galaxy evolution.
Gravitational Wave Background: The frequent mergers of SMBHBs throughout cosmic history contribute to a stochastic gravitational wave background. Detecting and characterizing this background can provide unique insights into the merger history of SMBHs and the overall evolution of galaxies.
Testing Cosmological Models: The merger history of galaxies, and consequently the formation and evolution of SMBHBs, is sensitive to the underlying cosmological model. Studying SMBHBs can therefore provide independent constraints on cosmological parameters and help refine our understanding of the universe's large-scale structure formation.
The potential presence of a recent SMBHB in the Milky Way highlights the importance of studying our own galaxy as a laboratory for understanding broader astrophysical processes. It emphasizes the interconnectedness of galactic mergers, SMBH evolution, and the overall evolution of galaxies in the universe.