Stochastic Noncircular Motion and Outflows Driven by Magnetic Activity in the Galactic Bulge Region

TL;DR

This study uses magneto-hydrodynamical simulations to show that magnetic activity causes noncircular gas motions and outflows in the Galactic bulge, offering an alternative explanation to bar-driven models.

Contribution

The paper introduces a magnetic activity-driven model for gas dynamics in the Galactic center, highlighting the role of magnetic turbulence and pressure gradients in shaping observed features.

Findings

Magnetic fields reach >~ 0.5 mG in the bulge region.

Radial flows are driven by turbulent magnetic fields and pressure gradients.

Simulated PV diagrams show asymmetric parallelogram-shapes matching observations.

Abstract

By performing a global magneto-hydrodynamical simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic center region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches >~ 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. In addition, the magnetic pressure-gradient force also drives radial flows in a similar manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram-shape of the central molecular zone. This is a natural extension into the central few 100 pc of the magnetic activity, which is observed as molecular loops at radii from a few 100 pc to 1 kpc. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time-dependent, which we discuss from a viewpoint of the outflow from the bulge.