Cosmological Simulations with Resolved Supermassive Black Hole Binary Dynamics and Accretion

This thesis investigates the properties of galaxies and supermassive black hole (SMBH) binaries in cosmological zoom-in simulations. The focus of the study is twofold: first, the agreement between simulated galaxies and observed scaling relations is assessed, followed by an analysis of the impact of different mass accretion models on SMBH binaries. To achieve these objectives, numerical simulations were run using the GADGET-3 and KETJU codes on the Mahti supercomputer hosted at the Finnish IT Centre for Science (CSC). Hydrodynamical cosmological simulations provide essential constraints and improvements for galaxy formation and evolution models by enabling direct comparisons with observations. For reliable results, simulated galaxies must resemble those in the real Universe. Here, it is found that the galaxies produced by KETJU generally follow observed relations, such as the specific stellar angular momentum -- stellar mass relation and the evolution of the size-mass relation. However, the supernova feedback in the simulation is found to be too inefficient, particularly in low-mass galaxies, resulting in excessive star formation and, consequently, over-massive galaxies and SMBHs. This work also presents the first implementation of a new KETJU binary mass accretion model in a cosmological simulation. A new simulation run of the original simulation volume was performed to evaluate the impact of a more physically motivated model on the SMBH binary properties. SMBHs coalesce as a result of a three-phased binary merger process, during which they interact with their surroundings via gas accretion and feedback. The results demonstrate that the new binary accretion model is necessary in order to resolve the SMBH binary evolution more accurately. In comparison with the old single accretion model, the binary accretion model produces more physically motivated mass ratios of the SMBHs by applying preferential accretion on to the secondary black hole. Additionally, it eliminates discontinuities in the mass accretion rates and black hole properties during the binary phase and prevents excessive star formation by maintaining gas densities below the star formation threshold. Contrary to previous expectations, the differences produced by the two models are also seen in gas-poor binaries. We find that fixing the binary eccentricity significantly reduces the discrepancies between different models. Overall, this study highlights the need for more complex SMBH accretion and feedback models to fully capture the co-evolution of galaxies and SMBH binaries in a cosmological context.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2025

Author dc:creator

• Soininen, Sonja

• galaxies
• simulation
• black holes
• astrophysics

Statement dc:rights

• CC BY 4.0

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/595543

OAI identifier oai:identifier

oai:helda.helsinki.fi:10138/595543