Global three-dimensional magentohydrodynamic simulations of galactic gaseous disks. I. amplification of mean magnetic fields in an axisymmetric gravitational potential

We carried out global three-dimensional resistive magnetohydrodynamic simulations of galactic gaseous disks to investigate how the galactic magnetic fields are amplified and maintained. We adopt a steady axisymmetric gravitational potential given by Miyamoto & Nagai and Miyamoto et al. As the initial condition, we assume a warm (T ∼ 10⁵ K) rotating gas torus centered at ω̄ = 10 kpc threaded by weak azimuthal magnetic fields. Numerical results indicate that in differentially rotating galactic gaseous disks, magnetic fields are amplified due to magnetorotational instability and magnetic turbulence develops. After the amplification of magnetic energy saturates, the disk stays in a quasi-steady state. The mean azimuthal magnetic field increases with time and shows reversals with a period of 1 Gyr (2 Gyr for a full cycle). The amplitude of B_φ near the equatorial plane is B_φ ∼ 1.5 μG at ω̄ = 5 kpc. The magnetic fields show large fluctuations whose standard deviation is comparable to the mean field. The mean azimuthal magnetic field in the disk corona has a direction opposite to the mean magnetic field inside the disk. The mass accretion rate driven by the Maxwell stress is ∼10^-3 M _⊙ yr^-1 at ω̄ = 2.5 kpc when the mass of the initial torus is ∼5 × 10⁸ M_⊙. When we adopt an absorbing boundary condition at r = 0.8 kpc, the rotation curve obtained by numerical simulations almost coincides with the rotation curve of the stars and the dark matter. Thus, even when magnetic fields are not negligible for gas dynamics of a spiral galaxy, the galactic gravitational potential can be derived from observations of the rotation curve using the gas component of the disk.