Formation of Galactic Center Magnetic Loops

Abstract

A survey of the molecular clouds in the Galaxy with the NANTEN mm telescope has discovered molecular loops in the galactic center region. They show monotonic gradients of the line-of-sight velocity along the loops and large velocity dispersions towards their foot-points. It is suggested that these loops can be explained in terms of a buoyant rise of magnetic loops due to a Parker instability. We carried out global three-dimensional magneto-hydrodynamic simulations of the gas disk in the galactic center. The gravitational potential was approximated by an axisymmetric potential proposed by Miyamoto and Nagai (1975, PASJ, 27, 533). At the initial state, we assumed a warm ($\sim$ 10$^4$K) gas torus threaded by azimuthal magnetic fields. Self-gravity and radiative cooling of the gas were ignored. We found that buoyantly rising magnetic loops are formed above the differentially rotating, magnetically turbulent disk. By analyzing the results of global MHD simulations, we identified individual loops, about 180 in the upper half of the disk, and studied their statistical properties, such as their length, width, height, and velocity distributions along the loops. The typical length and height of a loop are 1 kpc and 200 pc, respectively. The line-of-sight velocity changes linearly along a loop, and shows large dispersions around the foot-points. Numerical results indicate that loops emerge preferentially from the region where the magnetic pressure is large. We argue that these properties are consistent with those of molecular loops discovered by NANTEN.