Magnetic Flux Accumulation at the Galactic Center and Its Implications for the Strength of the Pregalactic Magnetic Field

Abstract

We study the inÑow of disk gas toward the Galactic center during the lifetime of the Galaxy and its e †ect on magnetic Ðeld lines frozen-in to the interstellar plasma. While compression leads to a large ampliÐcation of the "" vertical ÏÏ magnetic Ðeld (pointing perpendicular to the disk), ambipolar di †usion efficiently removes from the disk magnetic Ñux components oriented parallel to the Galactic plane. Turbulent interchange motions of nearly parallel vertical Ðeld lines at the Galactic center enhance the efficiency of magnetic reconnection of neighboring regions of oppositely directed vertical Ðeld. This suggests that the sign of the present-day vertical Ðeld at the Galactic center is uniform. If the Galactic-center Ðeld originates in the entrainment of a pregalactic Ðeld in radially inÑowing interstellar plasma, then B 0 observations of the vertical Ñux through the central 200 pc of our Galaxy yield a measure of the prega-lactic Ðeld that depends on the total mass accreted into the central 200 pc during the GalaxyÏs lifetime. If this mass is 3 ] 109 and if the surface density of disk gas is roughly constant over the lifetime of the M _ Galaxy, then G, regardless of the angle of the pregalactic Ðeld with respect to the Galac-B 0 Z 2 ] 10~7 tic plane. The abundance of mechanisms for radial accretion of disk gas suggests that strong magnetic Ðelds should be a generic feature of the centers of spiral galaxies. We also note that cosmic-ray conÐne-ment in the strong vertical Ðeld at the Galactic center is expected to be poor.