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
($R\lesssim120$~pc) and outer ($120\lesssim R\lesssim450$~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 $\infty$-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) down to the CMZ ($R\simeq200$~pc) of the order of
$1\rm\,M_\odot\,yr^{-1}$, consistent with observational determinations. (4)
Supernova feedback can drive an inflow from the CMZ inwards towards the
circumnuclear disc of the order of $\sim0.03\,\rm M_\odot\,yr^{-1}$. (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 ($R\lesssim30$~pc) to the Galactic
centre, but are also connected to the larger-scale streams at $R\gtrsim100$~pc.
