Massive black hole and gas dynamics in galaxy nuclei mergers. I. Numerical implementation

TL;DR

This paper introduces a new adaptive mesh refinement criterion that maintains high resolution around massive black holes during galaxy mergers, reducing numerical artifacts and improving orbit accuracy in simulations.

Contribution

A novel refinement strategy for AMR codes that preserves resolution around massive particles, enhancing the physical fidelity of black hole dynamics in merger simulations.

Findings

The new refinement criterion prevents spurious orbit perturbations.

High-resolution regions accurately track black hole orbits.

The method preserves physical processes during violent galaxy mergers.

Abstract

Numerical effects are known to plague adaptive mesh refinement (AMR) codes when treating massive particles, e.g. representing massive black holes (MBHs). In an evolving background, they can experience strong, spurious perturbations and then follow unphysical orbits. We study by means of numerical simulations the dynamical evolution of a pair MBHs in the rapidly and violently evolving gaseous and stellar background that follows a galaxy major merger. We confirm that spurious numerical effects alter the MBH orbits in AMR simulations, and show that numerical issues are ultimately due to a drop in the spatial resolution during the simulation, drastically reducing the accuracy in the gravitational force computation. We therefore propose a new refinement criterion suited for massive particles, able to solve in a fast and precise way for their orbits in highly dynamical backgrounds. The new refinement criterion we designed enforces the region around each massive particle to remain at the maximum resolution allowed, independently upon the local gas density. Such maximally-resolved regions then follow the MBHs along their orbits, and effectively avoids all spurious effects caused by resolution changes. Our suite of high resolution, adaptive mesh-refinement hydrodynamic simulations, including different prescriptions for the sub-grid gas physics, shows that the new refinement implementation has the advantage of not altering the physical evolution of the MBHs, accounting for all the non trivial physical processes taking place in violent dynamical scenarios, such as the final stages of a galaxy major merger.