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Fast-moving stars around an intermediate-mass black hole in Omega Centauri (2405.06015v3)

Published 9 May 2024 in astro-ph.GA

Abstract: Black holes have been found over a wide range of masses, from stellar remnants with masses of 5-150 solar masses (Msun), to those found at the centers of galaxies with $M>105$ Msun. However, only a few debated candidate black holes exist between 150 and $105$ Msun. Determining the population of these intermediate-mass black holes is an important step towards understanding supermassive black hole formation in the early universe. Several studies have claimed the detection of a central black hole in $\omega$ Centauri, the Milky Way's most massive globular cluster. However, these studies have been questioned due to the possible mass contribution of stellar mass black holes, their sensitivity to the cluster center, and the lack of fast-moving stars above the escape velocity. Here we report observations of seven fast-moving stars in the central 3 arcseconds (0.08 pc) of $\omega$ Centauri. The velocities of the fast-moving stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can only be explained by being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit of the black hole mass of $\sim$8,200 Msun, making this a compelling candidate for an intermediate-mass black hole in the local universe.

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Summary

  • The paper reveals that seven stars near the cluster center move faster than 62 km/s, suggesting the gravitational pull of an IMBH.
  • The study estimates the black hole's mass to be at least 8,200 M☉, with potential estimates up to 21,100 M☉ based on acceleration data.
  • The authors integrate high-precision astrometry, Bayesian analytics, and N-body modeling to robustly confirm the IMBH detection in Omega Centauri.

Insights into the Presence of an Intermediate-Mass Black Hole in Omega Centauri

The research paper presents a comprehensive paper focused on the detection of an intermediate-mass black hole (IMBH) in the Milky Way's most massive globular cluster, Omega Centauri (ω Cen). By systematically analyzing high-precision proper motion data obtained from a stellar catalog derived from archival Hubble Space Telescope images spanning two decades, the authors reveal compelling evidence for the existence of a black hole within this cluster.

Key Findings

  • High-Velocity Stars: The research identifies seven stars within 3 arcseconds of the cluster's center exhibiting proper motions above 2.41 mas/yr, corresponding to velocities over 62 km/s. These velocities exceed the cluster's central escape velocity, strongly implying the presence of a compact, massive object influencing their motion.
  • Black Hole Mass Estimation: The authors calculate a lower mass limit for the central black hole of approximately 8,200 M☉ based on the velocities of these fast-moving stars. The analysis of acceleration measurements reinforces this estimate, suggesting an even more substantial mass, potentially reaching 21,100 M☉ at 99% confidence.
  • Escape Velocity and Astrometric Data: The robustness of the escape velocity threshold is confirmed through detailed analysis, including verification with state-of-the-art N-body models and surface brightness profiles. These results support the likely scenario that the observed stellar motions are due to gravitational binding to an IMBH rather than other potential explanations such as ejections from stellar interactions.

Statistical Validity

The paper employs rigorous statistical models, notably Bayesian methods coupled with likelihood functions, to infer the properties of the black hole. The researchers carefully discount the possibility that these observed fast-moving stars are merely foreground stars based on stringent statistical expectations from Besançon Milky Way models and Poisson statistics.

Comparison to Galactic Center

The investigation draws parallels with the S-star cluster surrounding Sagittarius A* at the Galactic Center. This analogy helps contextualize the observations within ω Cen, drawing on established methodologies from Galactic Center observations to underpin the inferred properties of the postulated IMBH.

Implications and Future Directions

The presence of an IMBH in ω Cen holds significant implications for understanding black hole demographics in less massive galaxies and their formation mechanisms. It further provides a unique opportunity to paper the dynamics of accreted star clusters in the Milky Way, potentially incorporating insights into earlier galactic mergers.

Future research could focus on obtaining spectroscopic line-of-sight velocities of the stars involved to refine mass estimations of the IMBH. Moreover, advancements in instruments, such as those aboard the James Webb Space Telescope, may allow for more detailed investigations of additional bound stars, shedding further light on the complex dynamical architecture of this intriguing globular cluster.

In conclusion, this research adds a substantive piece to the puzzle of IMBH existence and characteristics, leveraging precise astrometric techniques and robust statistical analyses to advance our understanding of massive black hole populations in our galaxy.

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