Simulations of the MilkyWay’s central molecular zone - I. Gas dynamics

We use hydrodynamical simulations to study the MilkyWay’s central molecular zone (CMZ), i.e. the star-forming nuclear ring at Galactocentric radii R . 200 pc. The simulations comprise the gas flow in a Milky Way barred potential out to R = 5 kpc, which is necessary in order to capture the large-scale environment in which the CMZ is embedded and with which it is strongly interacting through the bar-driven inflow. The simulations also include a nonequilibrium time-dependent chemical network, gas self-gravity, and a sub-grid model for starformation and supernova feedback, all while reaching sub-parsec resolution in the densest regions. Our main findings are as follows: (1) The distinction between inner (R . 120 pc) and outer (120 . R . 450 pc) CMZ that is sometimes proposed in the literature is unnecessary. Instead, the CMZ is best described as single structure, namely a star-forming ring with outer radius R ' 200 pc which is interacting directly with the dust lanes that mediate the bar-driveninflow. (2) This accretion can induce a significant tilt of the CMZ out of the plane. A tilted CMZ might provide an alternative explanation to the ¥-shaped structure identified in Herschel data by Molinari et al. 2011. (3) The bar in our simulation efficiently drives an inflow from the Galactic disc (R ' 3 kpc) down to the CMZ (R ' 200 pc) of the order of 1M yr􀀀1, consistent with observational determinations. (4) Self-gravity and supernovae feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of 0:03M yr􀀀1.(5) We give a new interpretation for the 3D placement of the 20 and 50 km s􀀀1 clouds, according to which they are close (R . 30 pc) to the Galactic centre, but are also connected to the larger-scale streams at R & 100 pc.