Navonil Saha   (India)

navonil.saha @ uni-heidelberg.de

The Dynamics of Supermassive triple systems in a cosmological context

Throughout its lifetime, a massive galaxy typically experiences multiple major merger events with other galaxies and it is well established that Supermassive Black Holes (SMBHs) are present in the centers of these vast majority of galaxies and they grow together with their central SMBHs. In the post-merger phase of galaxy evolution, the SMBHs are sinking to the new galactic nucleus by dynamical friction and may form a binary or a multiple system. The timescales of SMBH binary mergers cover a large range of 50 Myr – 1 Gyr or longer. Full cosmological simulations still cannot resolve the formation and evolution of bound SMBH systems in the galaxy merger remnants. Thus it is important to perform high resolution N-body re-simulations to predict the outcome of these SMBH mergers and to find out the impact of the triaxial structure and of the central density profile of the host galaxies on the formation time of a hard SMBH binary in triple merger systems. It is also interesting to check what is the final outcome of SMBH triple systems and how long are the merger timescales.

Earlier cosmological simulations of triple SMBH systems were performed using IllustrisTNG-100 and ROMULUS25 with very high resolution. In the first case, none of the systems formed a bound SMBH binary and they stalled at 0.1-1 kpc separation. In the second case, all three triple systems finally formed a hard binary. In this project we plan to improve the initial conditions for the triple SMBH simulations by creating triaxial initial conditions using AGAMA and apply it to ROMULUS25 simulation. Additionally, we will search for potential triple SMBH merger systems in the high resolution IllustrisTNG-50 simulation with 8 times higher mass resolution in order to extend our sample.

For the early evolution of the global galaxy merger phase and the decay of the SMBHs due to dynamical friction, we will use the BONSAI2 tree code with ~10 million particles for each galaxy. In the later phase of hardening of the SMBH binaries by stellar encounters, we will switch to the φ-GPAPE-hybrid code in order to resolve the 3-body interactions correctly. Then, we will calculate the merger times and derive the expected GW signal of the merging SMBHs. Finally, we will use the direct φ-GPU code for validation of critical phases, when the merger remnant still shows too much sub-structure, which cannot be properly caught by the φ-GPAPE-hybrid code. With our sample, we expect a more realistic outcome of the triple SMBH interactions and a measurement of the delay of the merger times compared to the original cosmological simulations.

Supervisor:    Andreas Just  (ARI)