MAGNETOHYDRODYNAMIC SIMULATIONS OF DISK GALAXY FORMATION: THE MAGNETIZATION OF THE COLD AND WARM MEDIUM

Abstract

We modeled the formation and early evolution of disk galaxies with a magnetized interstellar medium using magnetohydrodynamic (MHD) adaptive mesh refinement simulations. For a 1010 M☉ halo with initial Navarro–Frenk–White dark matter and gas profiles, we impose a uniform 10−9 G magnetic field and follow its collapse, disk formation and evolution up to 1 Gyr. We find that the initial magnetic fields are quickly amplified by the differentially rotating turbulent disk with the amplification rate roughly one e-folding per orbit. After the initial rapid amplification lasting ∼500 Myr, subsequent field amplification appears self-regulated. The field strengths in the self-regulated regime have similar strength as the observed fields in the Milky Way galaxy both in the warm and the cold H i phases. Since supernova explosions, which we neglected in our current model, are likely to further amplify the magnetic field, our calculation suggests that Milky Way strength magnetic field might be common in high redshift disk galaxies. After saturation, highly magnetized material also begins to form above and below the disk, which may affect subsequent galaxy evolution, especially mergers, significantly. The global azimuthal magnetic fields reverse at different radii and the amplitude declines as a function of radius of the disk. We also find that magnetic force can provide further support in the cold gas and lead to a decline of the amount of cold gas at high density which may lead to a decline in the star formation rate.