Stochastic non-circular motion and outflows driven by magnetic activity in the Galactic bulge region

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 pressuregradient 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.

A theoretical explanation for the Central Molecular Zone asymmetry

“…Near the Galactic Centre (|l| 5 • ) there are several features that span a wide velocity range (∼ 100 km s −1 or more) while being confined to a narrow longitude range (e.g. Liszt 2006, see also Fukui et al 2006;Suzuki et al 2015). Bania Clump 2 is the prototypical example of such vertical features (Stark & Bania 1986).…”

Vertical flows and structures excited by magnetic activity in the Galactic center region

“…Such a strong magnetic field is expected to affect the gas dynamics near the Galactic center. Suzuki et al (2015) performed a global MHD simulation focusing on the Galactic bulge region under the axisymmetric Galactic potential (Miyamoto & Nagai 1975) by modifying the three dimensional MHD simulation code originally developed for protoplanetary disks (Suzuki & Inutsuka 2009;2014;Suzuki et al2010). In this proceedings paper, we investigated vertical flows and magnetic structure obtained from the numerical data, which were not analyzed in detail in Suzuki et al (2015).…”

Magnetic activity in the Galactic Centre region – fast downflows along rising magnetic loops

“…We adopt the numerical dataset in the GC region from a 3D-MHD simulation by Suzuki et al (2015). This simulation was performed by solving ideal MHD equations under an axisymmetric potential as external field; the Galactic gravitational sources consist of three components: the supermassive black hole (SMBH) whose mass is 4.4 × 10 6 M (Genzel et al 2010) at the GC (component i = 1), a stellar bulge (i = 2) and a stellar disc (i = 3).…”

“…where R and z are radius and height in the cylindrical coordinates, respectively, and M i , a i and b i are presented in Table 1. Suzuki et al (2015) assumed locally isothermal gas, namely the temperature is spatially dependent but constant with time. At each location, an equation of state for ideal gas,…”

“…With the background given above, our aim of the present paper is to pursue detailed gas motion by using the simulation data of Suzuki et al (2015). Our focus is particularly on the vertical motion of the gas.…”