Abstract

Contributed Talk - Splinter NonThermalAccel   (MW-1250)

The Dynamical Evolution of Triple Supermassive Black Holes in Triaxial Galactic Nuclei

Navonil Saha, Margarita Sobolenko, Peter Berczik, Andreas Just, Fazeel Mahmood Khan
Astronomisches Rechen-Institut, Universität Heidelberg

The hierarchical nature of galaxy formation in the ΛCDM cosmological framework naturally leads to repeated galaxy mergers and, consequently, to the presence of multiple supermassive black holes (SMBHs) in galactic nuclei. Understanding the dynamical evolution of these systems is essential for constraining the merger history of massive black holes and predicting the population of low-frequency gravitational-wave sources targeted by future observatories such as LISA. Furthermore, constraining the dynamical evolution of triple SMBHs provides the dynamical framework for future studies of accretion, AGN activity, and multi-messenger signatures associated with repeated galaxy mergers. While the evolution of SMBH binaries has been extensively studied, the long-term dynamics of triple SMBH systems, particularly in realistic non-spherical stellar environments, remain comparatively unexplored. In this work, we investigate the role of triaxiality in shaping the dynamical evolution of cosmologically motivated triple SMBH systems extracted from the ROMULUS25 cosmological simulation and embedded in dense triaxial stellar backgrounds using high-resolution direct N-body simulations. We explore a range of orbital configurations and host galaxy shapes, following the evolution from the galactic inspiral phase through the formation and hardening of SMBH binaries down to sub-parsec separations. Using the measured hardening rates, we estimate the subsequent coalescence timescales of the resulting binaries. Across all explored configurations, the two most massive black holes efficiently form a hard SMBH binary that is expected to merge within a Hubble time, while the third SMBH either remains on a wider galactic orbit or settles into a long-lived hierarchical triple configuration. We further analyse the structural evolution of the stellar remnant and find that the initial triaxiality of the host galaxy has only a modest influence on the final dynamical outcome of the triple systems. These results provide new constraints on the dynamical pathways leading to SMBH mergers in realistic galactic nuclei and improve our understanding of the gravitational-wave source population expected from hierarchical galaxy formation.