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 $R\sim 5\kpc$. Our main findings are as follows: (1) The distinction between inner ($R\lesssim 120$~pc) and outer ($120\lesssim R\lesssim 450$~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 $R\simeq 200$~pc which includes the 1.3${}^{\circ }$ 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 $\mathrm{\infty }$-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 ($R\simeq 3$kpc)downtotheCMZ($R\simeq200$pc)oftheorderof$1\rm,M_\odot,yr^{-1}$, consistent with observational determinations. (4) Supernova feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of $\sim 0.03{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. (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 ($R\lesssim 30$~pc) to the Galactic centre, but are also connected to the larger-scale streams at $R\gtrsim 100$~pc.