Magnetohydrodynamic Simulations of Disk Galaxy Formation: the Magnetization of The Cold and Warm Medium

TL;DR

This study uses MHD simulations to explore how magnetic fields influence the formation and evolution of disk galaxies, revealing rapid initial amplification and self-regulation of magnetic fields that match observed galactic values.

Contribution

It demonstrates that initial weak magnetic fields are quickly amplified and self-regulate, significantly affecting star formation rates in simulated disk galaxies.

Findings

Magnetic fields are amplified within 500 Myr and reach self-regulated strengths.

The simulated magnetic field strengths match observed Milky Way values.

Magnetic forces contribute to the decline of star formation rates after saturation.

Abstract

Using magnetohydrodynamic (MHD) adaptive mesh refinement simulations, we study the formation and early evolution of disk galaxies with a magnetized interstellar medium. For a $10^{10}$ \msun halo with initial NFW 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. Comparing to a purely hydrodynamic simulation with the same initial condition, we find that a protogalactic field of this strength does not significantly influence the global disk properties. At the same time, the initial magnetic fields are quickly amplified by the differentially rotating turbulent disk. After the initial rapid amplification lasting $\sim500$ Myr, subsequent field amplification appears self-regulated. As a result, highly magnetized material begin to form above and below the disk. Interestingly, the field strengths in the self-regulated regime agrees well with the observed fields in the Milky Way galaxy both in the warm and the cold HI phase and do not change appreciably with time. Most of the cold phase shows a dispersion of order ten in the magnetic field strength. The global azimuthal magnetic fields reverse at different radii and the amplitude declines as a function of radius of the disk. By comparing the estimated star formation rate (SFR) in hydrodynamic and MHD simulations, we find that after the magnetic field strength saturates, magnetic forces provide further support in the cold gas and lead to a decline of the SFR.