Magnetic flux accumulation at the galactic center and its implications for the strength of the pregalactic magnetic field

We study the inflow of disk gas toward the Galactic center during the lifetime of the Galaxy and its effect on magnetic field lines frozen-in to the interstellar plasma. While compression leads to a large amplification of the "vertical" magnetic field (pointing perpendicular to the disk), ambipolar diffusion efficiently removes from the disk magnetic flux components oriented parallel to the Galactic plane. Turbulent interchange motions of nearly parallel vertical field lines at the Galactic center enhance the efficiency of magnetic reconnection of neighboring regions of oppositely directed vertical field. This suggests that the sign of the present-day vertical field at the Galactic center is uniform. If the Galactic-center field originates in the entrainment of a pregalactic field B₀ in radially inflowing interstellar plasma, then observations of the vertical flux through the central 200 pc of our Galaxy yield a measure of the pregalactic field that depends on the total mass accreted into the central 200 pc during the Galaxy's lifetime. If this mass is 3 × 10⁹ M⊙ and if the surface density of disk gas is roughly constant over the lifetime of the Galaxy, then B₀ ≳ 2 × 10^-7 G, regardless of the angle of the pregalactic field with respect to the Galactic plane. The abundance of mechanisms for radial accretion of disk gas suggests that strong magnetic fields should be a generic feature of the centers of spiral galaxies. We also note that cosmic-ray confinement in the strong vertical field at the Galactic center is expected to be poor.