Similarity between the Molecular Loops in the Galactic Center and the Solar Chromospheric Arch Filaments

Abstract

Abstract We carried out two-dimensional magnetohydrodynamic simulations of the Galactic gas disk to show that the dense loop-like structures discovered by the Galactic center molecular cloud survey using the NANTEN 4-m telescope can be formed by a buoyant rise of magnetic loops due to the Parker instability. At the initial state, we assumed a gravitationally stratified disk consisting of a cool layer ($T$$\sim$ 10$^{3} $K), a warm layer ($T$$\sim$ 10$^{4} $K), and a hot layer ($T$$\sim$ 10$^{5} $K). The simulation box was a local part of the disk containing the equatorial plane. The gravitational field was approximated by that of a point mass at the Galactic center. The self-gravity, and the effects of the Galactic rotation were ignored. Numerical results indicate that the length of the magnetic loops emerging from the disk is determined by the scale height of the hot layer ($\sim$ 100 pc at 1 kpc from the Galactic center). The loop length, velocity gradient along the loops, and large velocity dispersions at their foot points are consistent with the NANTEN observations. We also show that the loops become top-heavy when the curvature of the loop is sufficiently small, so that the rising loop accumulates the overlying gas faster than sliding it down along the loop. This mechanism is similar to that in the formation of solar chromospheric arch filaments. The molecular loops emerge from the low-temperature layer just like the dark filaments observed in the H$\alpha$ image of the emerging flux region of the Sun.