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Astron. Astrophys. 345, 787-812 (1999)

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8. Conclusion

This paper presents the first fully self-consistent gas flow models of the inner Milky Way, obtained by incorporating an SPH component with [FORMULA] particles in realistic symmetry-free barred 3D N-body simulations with nearly [FORMULA] total number of particles. The axisymmetric initial conditions of the simulations are chosen to evolve spontaneously in a barred configuration compatible with the COBE/DIRBE K-band constraints, according to criteria based on a set of lower resolution pure stellar dynamical simulations realised in a precedent paper. The stellar bar rotates with its intrinsic natural pattern speed and the gas component, gently released from its axisymmetric configuration after the formation of the bar, freely interacts with the live stellar arms.

The density centre of the stellar bar becomes unstable and oscillates around the centre of mass with an amplitude of several 100 pc and a frequency of [FORMULA] km s-1 kpc- 1. The gas flow, contrary to other bisymmetric hydro simulations in fixed rotating barred potentials, never reaches a quasi-steady state and can delineate strong asymmetries in its spiral structure, not necessarily induced by the lopsided stellar distribution.

Some models selected from the simulations account for many features seen in the HI and CO longitude-velocity distributions within the Galactic bar and surrounding regions, and provide a very powerful guide to understand the inner structure of the Milky Way. The Galactic bar is inclined by [FORMULA] relative to the [FORMULA] line, has a corotation radius of [FORMULA] kpc, a related pattern speed [FORMULA] km s- 1 kpc-1, and a face-on axis ratio [FORMULA]. As in most early-type barred spirals, offset dustlanes are leading the bar major axis. Their gaseous traces in the observed [FORMULA] diagrams correspond to the connecting arm (near-side branch) and another feature near [FORMULA] (far-side branch). These dustlanes are the loci of strong shearing shocks with velocity changes up to 200 km s-1 across, and are located closer to the bar major axis than the leading edges of the cusped [FORMULA] orbit. The peaks in the terminal velocity curves at [FORMULA] are produced by the post-shock gas and not by the [FORMULA] trace of the latter orbit, which has velocity peaks at larger absolute longitude and with lower velocity amplitude. The near-side branch of the dustlanes lies below the Galactic plane ([FORMULA]) and the other branch, which is seen almost end-on, above it. Their maximum departure from the plane amounts to more than 100 pc.

The 3-kpc and 135-km s-1 arms are the inner prolongations of spiral arms in the disc, each passing close to one bar end and joining by a large bow around the nuclear ring/disc the dustlane in the other side of the bar, at galactocentric distances [FORMULA] kpc and 1.8 kpc respectively. The 135-km s-1 arm is associated with larger forbidden velocities because of its deeper location in the central potential well.

Velocity-elongated features in the observed CO [FORMULA] diagrams "below" the connecting arm, i.e. at similar longitudes but lower velocities than this arm, are interpreted as gas lumps which are just about to cross the shock front layer of the near-side dustlane. A robust example of such a feature is given by the vertical feature near [FORMULA]. Bania's clump 2 complex is a more controversial candidate because its mean Galactic latitude differs by roughly half a degree from that of the connecting arm at same longitude and because of its substantial mass. Finally, Bania's clump 1 complex is the part of the 135-km s-1 arm which strikes the far-side dustlane with a considerable incident velocity of order 100 km s-1. A fraction of its gas is directly absorbed by the dustlane, producing the apparent positive velocity trace of the dustlane, another fraction is gliding outwards along the dustlane, but the bulk of the clump is probably not intercepted by the dustlane.

The interpretation of the main observed [FORMULA] features given in this paper are by far the most plausible that can be deduced from our simulations. However, these simulations may still significantly depart from reality and hence the unicity of our interpretation cannot be guaranteed. For instance, if a large amount of gas is concentrated in the centre, a nuclear fast rotating bar may temporarily form and lead to a different understanding of the very complex innermost gas dynamics.

Some mpeg movies of the gas flow in simulation l10, including live [FORMULA] diagrams, are available on the web at http://obswww.unige.ch/~fux .

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999