KETJU · Post-Newtonian modelling of the dynamics of supermassive black holes in galactic-scale hydrodynamical simulations (KETJU)

Supermassive black holes (SMBHs) with masses in the range ~10^6-10^10 M⊙ are found at the centres of all massive galaxies in the Local Universe. In the ΛCDM picture of structure formation galaxies grow bottom-up through mergers and gas accretion, leading to multiple SMBHs in the same stellar system. Current simulation codes are unable to resolve in a single simulation the full SMBH merging process, which involves dynamical friction, three-body interactions and finally gravitational wave (GW) emission. KETJU will provide a significant breakthrough in SMBH research by following for the first time accurately global galactic-scale dynamical and gaseous astrophysical processes, while simultaneously solving the dynamics of SMBHs, SMBH binaries and surrounding stellar systems at sub-parsec scales. Our code KETJU (the word for 'chain' in Finnish) is built on the GADGET-3 code and it includes regions around every SMBH in which the dynamics of SMBHs and stellar particles is modelled using a non-softened Post-Newtonian algorithmic chain regularisation technique. The remaining simulation particles far from the SMBHs are evolved using softened GADGET-3. Using KETJU we can study at unprecedented accuracy the dynamics of SMBHs to separations of ~10 Schwarzschild radii, the formation of cores in massive galaxies, the formation of nuclear stellar clusters and finally provide a realistic prediction for the amplitude and frequency distribution of the cosmological gravitational wave background. The UH theoretical extragalactic team is ideally suited for this project, as it has an unusually versatile background in modelling the dynamics, feedback and merging of SMBHs. KETJU is also particularly timely, as the spectacular direct detection of GWs in 2016 is paving the way for a new era in gravitational wave astronomy. Future space-borne GW observatories, such as the European Space Agency's LISA, require accurate global GW predictions in order to fully realise their science goals.