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

Periodic Reporting for period 3 - KETJU (Post-Newtonian modelling of the dynamics of supermassive black holes in galactic-scale hydrodynamical simulations (KETJU))

Période du rapport: 2022-07-01 au 2023-12-31

In this project we study the formation and dynamical evolution of supermassive black holes that are found in the centres of all massive galaxies. These black holes although very massive are still spatially very small and it has been very challenging to resolve the dynamical interaction between the black holes and the stars in their host galaxies. In this project we have developed a new simulation code called KETJU that is able to simultaneously follow global galactic-scale processes, such as gas cooling, star formation and black hole physics, while solving in detail the dynamics of supermassive black hole binaries, and the surrounding stellar systems at very small separations. Critically our simulation code also includes so called Post-Newtonian force terms that model General relativistic phenomena, such as the emission of gravitational waves, which will be produced in abundance during the mergers of black holes.
Understanding the formation and impact of supermassive black holes on the evolution of galaxies is a key open question in astrophysical research. Black holes are the most extreme gravitationally bound objects in the Universe, and as such very good testbeds for gravitational physics in extreme conditions. Thus, understanding better the impact of black holes also on galactic scales could potentially lead to new insights in our fundamental understanding of physics and the Universe in which we live.
The overall objective of this project is to simulate at unprecedented accuracy the dynamical evolution of supermassive black holes and their surrounding stars in galaxies that are formed in a full cosmological setting. In doing so we will be able to produce very high precision predictions of the gravitational wave signal from merging supermassive black holes in gas-rich galaxies. This will important when making predictions for future space-borne gravitational wave observatories, such as the European Space Agency’s LISA mission, which is planned to be launched in the mid-2030s.
This work in this project is organized through five work packages (WPs). In WPA-1 we develop new numerical techniques that allow for simulations with much higher numbers of stellar particles close to the supermassive black holes. Crucially we have developed a new version of our KETJU code that allows simulating also supermassive black holes in gas-rich galaxies, which are expected to have strong levels of star formation in their centres. In WPA-2, which is also the most challenging part of the project, we develop new models that more accurately describe the accretion of gas onto binary black hole systems, as our runs, unlike previous simulations, are able to dynamically resolve the prolonged binary evolution of supermassive black holes. This work is currently still in progress, with the first KETJU black hole binary feedback model published later this year (2022). In WPs B1-B3 we apply the KETJU code to various astrophysical settings. Specifically in WPB-1 we have demonstrated that the formation of cores (i.e. regions with low stellar density) in massive galaxies is caused by complex three-body interactions in which the stars are ejected by the binary supermassive black hole. In WPB-2 we have performed for the first time a set of high-resolution cosmological simulations of forming galaxies that include gas physics, star formation, and an accurate description for black hole dynamics. Using these simulations, we have also made predictions for the strength of the gravitational wave signal from merging supermassive black holes in cosmologically forming galaxies. Finally, in WPB-3 we have studied the formation of massive stellar clusters in the mergers of gas-rich dwarf galaxies. Our simulations show that these typed of clusters can indeed form in such a setting and that they have properties that are in good agreement with the observed population of globular clusters found in the local universe.
This project promises to deliver for the very first time high-resolution cosmological galaxy formation simulations that are also able to resolve the dynamical evolution of binary supermassive black holes, including the final gravitational-wave driven merger phase. The KETJU code that is developed as part of this project enables gas-free merger simulations with up to 100 million particles per galaxy, which is almost two orders of magnitude beyond the capabilities of traditional direct N-body codes. Crucially the KETJU code also enables simulations that include full gas physics, star formation, and feedback from both supernovae and supermassive black holes. Typically, direct N-body codes do not include a gaseous component.

Using the KETJU code we will study the formation of cores in cosmologically forming massive galaxies for the first time, as we are able to now resolve both the gaseous component and the strong gravitational interactions between the supermassive black hole binary and the surrounding stars. We will also simulate a large cosmological volume that resolves the formation of thousands of galaxies, and now for the first time also includes detailed dynamical modelling of their supermassive black holes. This will enable us to produce predictions of the expected gravitational wave background at unprecedented fidelity. Finally, we will study the formation of massive stellar clusters in merging gas-rich galaxies and using KETJU resolve for the first time also their internal dynamics in a gas-rich environment.

This project is thus expected to produce several landmark results in dynamical and gravitational astrophysics, especially related to the dynamics of supermassive black holes, the emission of gravitational waves and the formation and evolution of massive stellar clusters.
Overview of a cosmological KETJU simulation, showing the different physical scales modelled.