The impact of magnetic fields on cosmological galaxy mergers. I: Reshaping gas and stellar discs

TL;DR

This study uses high-resolution MHD simulations to show that magnetic fields significantly influence the structural evolution of galaxy merger remnants, leading to more extended discs and spiral features compared to non-magnetic models.

Contribution

First cosmologically-consistent investigation of magnetic fields' effects on major galaxy mergers, revealing their role in shaping remnant morphology.

Findings

Magnetic fields lead to extended, spiral-rich galaxy remnants.

Hydrodynamic simulations produce more compact, ring-like remnants.

Resolution affects the amplification of magnetic fields and morphological outcomes.

Abstract

Mergers play an important role in galaxy evolution. In particular, major mergers are able to have a transformative effect on galaxy morphology. In this paper, we investigate the role of magnetic fields in gas-rich major mergers. To this end, we run a series of high-resolution magnetohydrodynamic (MHD) zoom-in simulations with the moving-mesh code Arepo and compare the outcome with hydrodynamic simulations run from the same initial conditions. This is the first time that the effect of magnetic fields in major mergers has been investigated in a cosmologically-consistent manner. In contrast to previous non-cosmological simulations, we find that the inclusion of magnetic fields has a substantial impact on the production of the merger remnant. Whilst magnetic fields do not strongly affect global properties, such as the star formation history, they are able to significantly influence structural properties. Indeed, MHD simulations consistently form remnants with extended discs and well-developed spiral structure, whilst hydrodynamic simulations form more compact remnants that display distinctive ring morphology. We support this work with a resolution study and show that whilst global properties are broadly converged across resolution and physics models, morphological differences only develop given sufficient resolution. We argue that this is due to the more efficient excitement of a small-scale dynamo in higher resolution simulations, resulting in a more strongly amplified field that is better able to influence gas dynamics.