Formation of Galactic Center Magnetic Loops

TL;DR

This paper presents 3D MHD simulations of magnetic loops in the Galactic center, explaining observed molecular loops as buoyant magnetic structures arising from Parker instability.

Contribution

The study provides the first global 3D MHD simulation of magnetic loop formation in the Galactic center, matching observed properties of molecular loops.

Findings

Magnetic loops of about 1 kpc length and 200 pc height are formed above the disk.

Line of sight velocity along loops shows linear variation and large dispersions at foot-points.

Loops emerge from regions with high magnetic pressure, consistent with observations.

Abstract

A survey for the molecular clouds in the Galaxy with NANTEN mm telescope has discovered molecular loops in the Galactic center region. The loops show monotonic gradients of the line of sight velocity along the loops and the large velocity dispersions towards their foot points. It is suggested that these loops are explained in terms of the buoyant rise of magnetic loops due to the Parker instability. We have carried out global three-dimensional magneto-hydrodynamic simulations of the gas disk in the Galactic center. The gravitational potential is approximated by the axisymmetric potential proposed by Miyamoto & Nagai (1975). At the initial state, we assume a warm (~ 10^4 K) gas torus threaded by azimuthal magnetic fields. Self-gravity and radiative cooling of the gas are 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 have 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. Typical length and height of a loop are 1kpc and 200pc, 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 magnetic pressure is large. We argue that these properties are consistent with those of the molecular loops discovered by NANTEN.