KETJU - resolving small-scale supermassive black hole dynamics in GADGET-4

We present in this work the new public version of the KETJU supermassive black hole (SMBH) dynamics module, as implemented into GADGET-4. KETJU adds a small region around each SMBH where the dynamics of the SMBHs and stellar particles are integrated using an algorithmically regularized integrator instead of the leapfrog integrator with gravitational softening used by GADGET-4. This enables modelling SMBHs as point particles even during close interactions with stellar particles or other SMBHs, effectively removing the spatial resolution limitation caused by gravitational softening in galaxy evolution simulations. KETJU also includes post-Newtonian (PN) corrections, which allows following the dynamics of SMBH binaries to sub-parsec scales. Systems with multiple SMBHs are also supported, with the code also including the leading non-linear cross terms that appear in the PN equations for such systems.

The paper demonstrates tests of the code showing that it correctly captures, at sufficient mass resolution, the sinking driven by dynamical friction and binary hardening driven by stellar scattering. Also presented is an example application demonstrating how the code can be applied to study the dynamics of SMBHs in mergers of multiple galaxies and the effect they have on the properties of the surrounding galaxy.

The full details of the study can be found in the article KETJU - resolving small-scale supermassive black hole dynamics in GADGET-4.