Galaxy mergers are a common occurrence in the universe. When galaxies merge, their central supermassive black holes (SMBHs) can also merge and form a supermassive black hole binary (SMBHB). The formation of such a binary is more efficient when the SMBHs are surrounded by a nuclear star cluster (NSC), a dense and massive stellar system. NSCs are present in most observed galaxies, with central densities that can reach up to 10^7, M⊙/pc^3 and masses that can be as large as a few 10^7, M⊙.
Detecting an SMBHB directly can be challenging. However, recent research conducted by Alessandra Mastrobuono-Battisti, Go Ogiya, Oliver Hahn, and others suggests that the shape, density profile, rotation, and velocity structure of an NSC can help identify nucleated galaxies that have gone through a merger and possibly formed a central SMBHB.
In their research, the team ran simulations that showed around 60% of the stars were on retrograde orbits. Additionally, the majority of the ejected stars came from the very central regions of their progenitor NSC. The team also found that the NSC initially hosting the most massive SMBH produced the largest number of hypervelocity stars (HVSs), and the number of HVSs strongly depended on the SMBH mass ratio and merger characteristics.
The study also revealed that the merger can contribute to the formation of the nuclear stellar disc of the galaxy. This process is efficient due to the combined effect of dynamical friction, stellar hardening, and the ouroboros effect, a drag force caused by the stars in the tidal streams of the NSCs.
While NSCs have been linked to the properties of their parent galaxy, their formation process is not yet clear. NSCs are thought to form through a mixture of in-situ star formation and dynamical friction-driven star cluster decay and mergers.
The research by Mastrobuono-Battisti, Ogiya, Hahn, and others has opened up new avenues for the detection of past binary SMBH mergers in nuclear star clusters. By examining the large-scale structural and dynamical properties of NSCs, it may be possible to identify nucleated galaxies that have recently gone through a merger that potentially led to the formation of a central SMBHB.
This research provides exciting insights into the formation and detection of SMBHBs in NSCs. The findings show that the merger can imprint signatures on the shape, density profile, rotation, and velocity structure of the NSC. Additionally, the number and kinematic properties of HVSs are a direct consequence of the SMBHB properties. Further research in this area may lead to a better understanding of the formation and evolution of galaxies and their central SMBHs.
Source: Mastrobuono-Battisti, A., Ogiya, G., Hahn, O., & Schultheis, M. (2023). Searching for clues of past binary supermassive black hole mergers in nuclear star clusters. https://doi.org/10.48550/arXiv.2303.12826