A 40-billion solar mass black hole in the extreme core of Holm 15A, the central galaxy of Abell 85 (1907.10608v2)

Abstract: Holm 15A, the brightest cluster galaxy (BCG) of the galaxy cluster Abell 85, has an ultra-diffuse central region, 2 mag fainter than the faintest depleted core of any early-type galaxy (ETG) that has been dynamically modelled in detail. We use orbit-based, axisymmetric Schwarzschild models to analyse the stellar kinematics of Holm 15A from new high-resolution, wide-field spectral observations obtained with MUSE at the VLT. We find a supermassive black hole (SMBH) with a mass of (4.0 +- 0.80) x 10^10 solar masses at the center of Holm 15A. This is the most massive black hole with a direct dynamical detection in the local universe. We find that the distribution of stellar orbits is increasingly biased towards tangential motions inside the core. However, the tangential bias is less than in other cored elliptical galaxies. We compare Holm 15A with N-body simulations of mergers between galaxies with black holes and find that the observed amount of tangential anisotropy and the shape of the light profile are consistent with a formation scenario where Holm 15A is the remnant of a merger between two ETGs with pre-existing depleted cores. We find that black hole masses in cored galaxies, including Holm 15A, scale inversely with the central stellar surface brightness and mass density, respectively. These correlation are independent of a specific parameterization of the light profile.

• The paper reports the detection of a 40-billion solar mass black hole in the center of Holm 15A, the most massive black hole found via direct dynamical detection using VLT/MUSE data.
• This exceptionally massive black hole is an outlier in common galaxy-SMBH scaling relations, suggesting a formation history involving major galaxy mergers with depleted cores.
• The findings provide insights into the formation of extreme galaxy cores and the crucial role of supermassive black holes in the evolution of massive galactic systems.

Analysis of a Supermassive Black Hole in Holm 15A in Abell 85

The paper "A 40-billion solar mass black hole in the extreme core of Holm 15A, the central galaxy of Abell 85" by Mehrgan et al. employs a sophisticated analysis to explore the properties of one of the most massive black holes discovered via direct dynamical detection. Situated at the center of Holm 15A, a Brightest Cluster Galaxy (BCG) within the galaxy cluster Abell 85, this supermassive black hole (SMBH) carries significant implications for our understanding of galaxy formation and evolution, particularly in massive elliptical galaxies.

Summary and Methodology

The research uses orbit-based, axisymmetric Schwarzschild models to explore the stellar kinematics of Holm 15A. Utilization of the MUSE instrument at the VLT provided high-resolution spectroscopic data critical for the precise determination of the SMBH mass, which is reported to be $(4.0 \pm 0.80) \times 10^{10} \ \si{M_{\odot}}$, making it the most massive black hole with a direct dynamical detection in the local universe. This mass is significantly higher than typical expectations given the galaxy's stellar mass and velocity dispersion, positioning Holm 15A as an anomalous outlier in galaxy SMBH scaling relations.

Results and Implications

This SMBH mass is indicative of a rich and complex formation history. The kinematic analysis revealed a tangential bias in stellar orbits within the central core, albeit less pronounced than in similar cored elliptical galaxies. The light profile of Holm 15A and its place among other core galaxies were also considered, showing consistency with the characteristics of other BCGs, such as NGC 1600 and NGC 4889. The findings support a formation narrative for Holm 15A rooted in significant merging activity. Comparisons with N-body simulations suggest that Holm 15A may be the product of a merger between two early-type galaxies that already featured depleted cores.

Core and Black Hole Scaling Relations

Despite hosting the most massive known SMBH, Holm 15A follows established correlations between core properties and black holes, such as between central stellar surface mass density and SMBH mass. These scaling laws have notable implications for understanding galaxy formation, especially within massive clusters like Abell 85. The paper underscores the potential of SMBHs as pivotal players in galaxy evolution narratives, driving the formation of central cores in massive elliptical galaxies via processes like binary black hole interactions.

Conclusions and Future Directions

The finding of such a massive SMBH within Holm 15A offers valuable insights into extreme instances of core formation and the role of SMBHs as central figures in galaxy evolution. The research invites further exploration into the connections between galactic core properties and SMBH masses, particularly within the framework of cosmological models and larger-scale structure formation. Moreover, these observations set the stage for future high-angular-resolution studies, potentially involving instruments like the Event Horizon Telescope, to empirically detail SMBH environments at spatial scales comparable to the gravitational sphere of influence.

In essence, this paper elevates our understanding of galaxy dynamics and the fundamental interplay between supermassive black holes and their host galaxies, emphasizing the complexities underpinning the formation and evolution of massive galactic systems.