Intermediate-mass black hole binary as an origin of super-massive black holes

Sammanfattning: Galactic nuclei contain super-massive black holes (SMBHs), nuclear stellar clusters (NSCs), or both. In the NSC, intermediate-mass black holes (IMBHs) are likely to reside and form highly eccentric binaries. Over time, the orbit of a binary could harden via interaction with surrounding low-mass stars. Eventually, it would be possible for the IMBHs to, via the emission of gravitational waves (GWs), merge into an SMBH seed that over time can grow and result in an SMBH in the galactic nuclei. The aim of this thesis is to establish under what circumstances mergers occur. I wish to track which encounters are most responsible for taking the binary from formation to merger and see if this can happen within a reasonable astrophysicalmerger time. This would shine a light on whether IMBH binary mergers are possible and, by extension, further motivate the attempted detections of GWs from these events. Moreover, it would reinforce IMBH binaries as a possible origin of SMBH seeds in the NSC. I aim to also investigate the importance of different parameters such as mass ratio and initial eccentricity to see what part they play in bringing the system to a point of merger. Multiple N-body simulations of an IMBH binary and a lower mass impactor were carried out via the TSUNAMI code. Its output was then utilized when tracking the evolution of the semi-major axis and eccentricity and how different encounters affected the binary. Besides that, the output was used when investigating the effect of altering previously mentioned parameters. The evolution of the system was put in relation to a Toy model and N-body run presented by Askar et al. (2021). Despite the simulations’ stochastic nature, it was found that the investigated IMBH binary significantly hardened during an encounter-driven evolution whilst staying noticeably eccentric. This allowed the binary to reduce its astrophysical merger time which enabled SMBH seed formation. But whilst this all qualitatively aligns with the findings of Askar et al. (2021), it was established that the assumptions embedded in the Toy model led it to exhibit an unrealistic eccentricity pumping. What mainly separated the here performed simulations from those in the Toy model was the assumption that each encounter affected the whole binary instead of only one constituent, as was the case in the Toy model. It was discussed how there was no obvious change in merger time from an altered initial eccentricity but how a lower impacting mass was less efficient in bringing the system to a merger. When allowing for wider encounters the results were somewhat ambiguous and require more investigation before any conclusions can be drawn. Each simulated encounter had a unique offset, velocity, and encounter time but to further elaborate on the finding of this thesis, assumptions such as the third mass being constant and the body’s point-like behaviour should be re-evaluated.