Magnetfeldverstärkung und Star-Bursts in verschmelzenden Galaxien
Zusammenfassung der Projektergebnisse
We explored the evolution of galactic magnetic fields during interactions and mergers of disk galaxies by means of numerical simulations. Observations and previous theoretical studies show that mergers play a significant role in the evolution of galaxies. Prior to our project, only very simple simulations of adiabatic gaseous disks without dark matter halos were carried out with grid-based codes. We accomplished a major step toward more realistic simulations of galaxy mergers by applying N-body dynamics to model the dark-matter halos associated with disk galaxies. We employed the magnetohydrodynamical adaptive mesh refinement code Enzo, including a novel subgrid-scale model for magnetohydrodynamical turbulence. This allowed us to compute and analyse magnetic field amplification induced by the tidal interactions between the galaxies. The methodology was developed and tested in simulations of isolated disks and then extended to colliding galaxies. Quite a number of numerical obstacles had to be tackled until we arrived at an equilibrium configuration of the magnetized gas disk and the surrounding dark-matter halo that is stable for a sufficiently long period to avoid spurious effects in simulations of interacting systems. In an elaborate parameter study, we varied the impact parameter (i. e., the normal distance of the trajectory of the inbound galaxy to the target galaxy) and the relative disk orientations. In each simulation, a maximal spatial resolution of 15 pc inside the gaseous disks was reached by means of adaptive mesh refinement in a computational box of 2 Mpc size. Such a large box is necessary to accommodate the dark-matter halos of both galaxies. The chosen setup poses a substantial challenge in terms of computational resources and pushes the capabilities of Enzo to its limits. We found pronounced peaks of the mean magnetic field strength during interaction phases and mergers, which are in agreement with observations. The shape and maximum of the peaks is not only sensitive to the impact parameter of the galaxies but also to disk orientations. An analysis of the field-generating electromotive force provided evidence of a mean-field dynamo acting on small length scales (a few 100 pc). However, we see that the magnetic field amplification is generally transient. This can be understood as a consequence of the transfer of magnetic field energy to the medium outside of the galaxies. The initially ordered toroidal fields of the disks evolve into highly turbulent structures, particularly in collision fronts and tidal bridges between galaxies, suggesting that interactions and mergers play an important role in the magnetization of the CGM. Moreover, we found that nearly central collisions result in elliptical galaxies in the post-merger phase. For a complete model of interacting galaxies, radiative cooling, star formation, and stellar feedback are essential components. However, it turned out that the numerical resolution required for such a model is not feasible in simulations with the Enzo code. This will be possible in the future, using a more advanced code with higher scalability.
