Abstract

Contributed Talk - Splinter SMBHs

Thursday, 10 September 2026, 15:05   (MW-0250)

The dynamical evolution of cosmological supermassive black hole systems triples in realistic galactic nuclei

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

Galaxy mergers in the ΛCDM cosmological framework naturally lead to the formation of multiple supermassive black holes (SMBHs) within galactic nuclei. Understanding the dynamical evolution of these systems is essential for constraining the assembly history of SMBHs, predicting merger rates for future gravitational wave observatories such as LISA, and interpreting observations of dual and multiple active galactic nuclei. Constraining the evolution of triple SMBH systems further provides the dynamical framework for future studies of accretion, AGN activity, and multi-messenger signatures associated with repeated galaxy mergers. Despite their importance, the long-term evolution of triple SMBHs in realistic, non-spherical galactic potentials remains largely unexplored. We investigate the evolution of cosmologically motivated triple SMBH systems extracted from the ROMULUS25 cosmological simulation and embedded in dense triaxial stellar environments using high-resolution direct N-body simulations. We follow the orbital evolution from the galactic inspiral phase through binary formation and hardening to sub-parsec separations for a range of host-galaxy triaxialities and orbital configurations. From the measured hardening rates, we estimate the subsequent coalescence timescales of the resulting SMBH binaries and assess the long-term fate of the third black hole. Across all explored configurations, the two most massive SMBHs efficiently form a hard binary that is expected to merge within a Hubble time, while the third SMBH either remains on a wider galactic orbit or evolves into a long-lived hierarchical triple configuration. We also find that the initial triaxiality of the host galaxy has only a modest influence on the final dynamical outcome of the system. Our results provide physically motivated predictions for the occurrence of close SMBH binaries, wandering SMBHs, and hierarchical triple systems in galactic nuclei, offering theoretical constraints that complement electromagnetic searches for multiple SMBHs and future low-frequency gravitational-wave observations. By connecting cosmological galaxy assembly with parsec-scale SMBH dynamics, this work helps bridge numerical modelling and the interpretation of next generation observational surveys and multi-messenger datasets.