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

Galaxy mergers are common processes in the Universe. As a large fraction of galaxies hosts at their centres a central supermassive black hole (SMBH), mergers can lead to the formation of a supermassive black hole binary (SMBHB). The formation of such a binary is more efficient when the SMBHs are embedded in a nuclear star cluster (NSC). NSCs are dense and massive stellar clusters present in the majority of the observed galaxies. Their central densities can reach up to 10(7) M-circle dot(3) and their masses can be as large as a few 10(7) M-circle dot. The direct detection of an SMBHB is observationally challenging. In this work, we illustrate how the large-scale structural and dynamical properties of an NSC can help to identify nucleated galaxies that recently went through a merger that possibly led to the formation of a central SMBHB. Our models show that the merger can imprint signatures on the shape, density profile, rotation, and velocity structure of the NSC. The strength of the signatures depends on the mass ratio between the SMBHs and on the orbital initial conditions of the merger. In addition, the number of hypervelocity stars produced in the mergers is linked to the SMBHB properties. The merger can also contribute to the formation of the nuclear stellar disc of the galaxy.

Automated summary

Galaxy mergers can lead to the formation of supermassive black hole binaries, especially when the black holes are embedded in a nuclear star cluster. This study demonstrates that the structural and dynamical properties of the cluster can be used to identify recently merged galaxies with potential central supermassive black hole binaries. Mergers can affect the shape, density profile, rotation, and velocity structure of the cluster, depending on the mass ratio and initial conditions of the merger. Additionally, mergers may contribute to the formation of the nuclear stellar disc in the galaxy.