Simulations of the Milky Way's central molecular zone - I. Gas dynamics

We use hydrodynamical simulations to study the Milky Way's central molecular zone (CMZ). The simulations include a non-equilibrium chemical network, the gas self-gravity, star formation, and supernova feedback. We resolve the structure of the interstellar medium at sub-parsec resolution while also capturing the interaction between the CMZ and the bar-driven large-scale flow out to R similar to 5 kpc. Our main findings are as follows: (1) The distinction between inner (R less than or similar to 120 pc) and outer (120 less than or similar to R less than or similar to 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 similar or equal to 200 pc which includes the 1.3 degrees complex and which is directly interacting with the dust lanes that mediate the bar-driven inflow. (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 infinity-shaped structure identified in Herschel data by Molinari et al. (3) The bar in our simulation efficiently drives an inflow from the Galactic disc (R similar or equal to 3 kpc) down to the CMZ (R similar or equal to 200 pc) of the order of 1 M o yr I , consistent with observational determinations. (4) Supernova feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of -0.03 M-circle dot 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 less than or similar to 30 pc) to the Galactic Centre, but are also connected to the larger scale streams at R greater than or similar to 100 pc.