# Searching for clues of past binary supermassive black hole mergers in nuclear star clusters

**Authors:** Alessandra Mastrobuono-Battisti, Go Ogiya, Oliver Hahn, Mathias Schultheis  
**arXiv:** [2303.12826](https://arxiv.org/abs/2303.12826)  
**Date:** 2023-03-22  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.12826v1/

## Key points

- Galaxy mergers can lead to the formation of a supermassive black hole binary (SMBHB) at the center of a galaxy
- The efficiency of SMBHB formation is increased when the central supermassive black holes are embedded in a nuclear star cluster (NSC)
- NSCs have central densities up to $10^7\,M_\odot/{\rm pc}^3$ and masses as large as several $10^7\,M_\odot$
- Structural and dynamical properties of an NSC can help identify nucleated galaxies that recently underwent a merger leading to the formation of an SMBHB
- Mergers imprint signatures on the shape, density profile, rotation, and velocity structure of NSCs
- These signatures depend on the mass ratio between SMBHs and their initial orbital conditions during merging
- Mergers contribute to forming nuclear stellar discs in galaxies
- The number of hypervelocity stars produced during mergers is linked to SMBHB properties
- Observations of NSCs' large-scale structural and dynamical properties can provide insights into past or ongoing binary black hole activity in galactic centers.

## Summary

Galaxy mergers are a common occurrence in the Universe and can lead to the formation of a supermassive black hole binary (SMBHB) at the center of a galaxy. The efficiency of SMBHB formation is increased when the central supermassive black holes (SMBHs) are embedded in a nuclear star cluster (NSC), which is a dense and massive stellar cluster present in most observed galaxies. NSCs have central densities up to $10^7\,M_\odot/{\rm pc}^3$ and masses as large as several $10^7\,M_\odot$. In this work by Alessandra Mastrobuono-Battisti et al., the authors illustrate how structural and dynamical properties of an NSC can help identify nucleated galaxies that recently underwent a merger leading to the formation of an SMBHB. The authors' models show that mergers imprint signatures on the shape, density profile, rotation, and velocity structure of NSCs. These signatures depend on the mass ratio between SMBHs and their initial orbital conditions during merging. Additionally, they suggest that mergers contribute to forming nuclear stellar discs in galaxies - flattened structures composed mainly of stars located near galactic centers. Furthermore, the number of hypervelocity stars produced during mergers is linked to SMBHB properties. Hypervelocity stars are ejected from galactic centers due to gravitational interactions with an SMBHB; thus their presence could indicate past or ongoing binary black hole activity. This study highlights how observations of NSCs' large-scale structural and dynamical properties can provide insights into past or ongoing binary black hole activity in galactic centers.