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

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1. Introduction

The gas kinematics near the Galactic plane, mainly observed in HI (Burton 1985; Kerr et al. 1986; Stark et al. 1992 and Hartmann & Burton 1997), 12CO (Dame et al. 1987; Oka et al. 1998b), 13CO and CS (Bally et al. 1987), is probably among the best tracer of the dynamical mass in the Galaxy. Under the assumption of circular motion in an axisymmetric potential, the tangent point method allows in principle to derive the rotation curve inside the solar circle and to recover the spatial location of the spiral arms (see the compilation of Vallée 1995).

However, this approach is limited for at least two reasons related to the data themselves. First, it is long known (Rougoor & Oort 1960) that the longitude-velocity ([FORMULA]) distribution of the Galactic gas reveals substantial diffuse and feature-like emission in the regions [FORMULA] and [FORMULA] close to the Galactic centre which are forbidden to pure circular motion, the most popular example being the 3-kpc arm. Second, bumps of about 7 km s-1 on the terminal velocity curves near the tangent points of the spiral arms reflect the gravitational perturbation of these arms on the gas flow and can propagate into distance errors of [FORMULA] kpc along the line of sight if modeled by circular motion (Burton 1992; Combes 1991).

Traditional interpretations of the "forbidden" velocities near the centre involve explosion induced expansion (van der Kruit 1971; van der Kruit et al. 1972; Cohen 1975; Oort 1977) or gas moving on elliptical orbits in either a spiral (Shane 1972; Simonson & Mader 1973) or a barred potential (Peters 1975; Cohen & Few 1976; Liszt & Burton 1980; Gerhard & Vietri 1986). In the last decade, the latter interpretation received strong support by direct evidence of a large scale stellar bar in the Milky Way from near-IR surface photometry (Blitz & Spergel 1991; Dwek et al. 1995; Binney et al. 1997), discrete source counts (Nakada et al. 1991 and Izumiura et al. 1994 for SiO masers, Weinberg 1992 and Nikolaev & Weinberg 1997 for AGB stars; Whitelock & Catchpole 1992 for Miras variables; Blitz 1993 for globular clusters; Sevenster 1996 for OH/IR stars; Stanek et al. 1997 for red clump stars) and large microlensing optical depths towards the bulge (Paczynski et al. 1994; Evans 1994; Zhao et al. 1996; Zhao & Mao 1996; Gyuk 1999; see Gerhard 1996 and Kuijken 1996 for reviews).

Many hydrodynamical simulations (Roberts et al. 1979; van Albada 1985; Mulder & Liem 1986; Athanassoula 1992; Englmaier & Gerhard 1997) have shown that the gas flow in a rapidly rotating barred potential is driven by strong shocks leading the bar major axis and followed downstream by enhanced gas densities. In external early-type (i.e. Sbc and earlier) barred spirals, these shocks are detected as offset dustlanes with very large velocity changes in the associated gas velocity field (e.g. Reynaud & Downes 1998; Laine et al. 1999).

Binney et al. (1991) have interpreted the gas kinematics near the Galactic centre approximating the gas streamlines by a sequence of closed [FORMULA] and [FORMULA] orbits (Contopoulos & Mertz-anides 1977), but such a model does not properly cares about the shocks. The hydro simulations designed for the Milky Way (Mulder & Liem 1986; Wada et al. 1994; Englmaier & Gerhard 1999; Weiner & Sellwood 1999) always assume rigid barred potentials and bisymmetry with respect to the Galactic centre, but some details in the HI and CO observations clearly betray important asymmetries: (i) about 3/4 of the molecular gas in the nuclear ring/disc lies at positive longitude, (ii) the 3-kpc arm and its far-side counterarm have very different absolute velocities at [FORMULA], and (iii) the positive and negative velocity peaks of bright emission, at [FORMULA] and [FORMULA] respectively, differ by roughly 50 km s-1 in absolute velocity, and the emission of the nuclear ring/disc is also shifted towards receding velocities. Moreover, as an SBbc galaxy (Sackett 1997), the Milky Way is also expected to present prominent dustlanes, but their gaseous traces have never been convincingly isolated in the HI and CO data.

This paper follows a first paper (Fux 1997, hereafter Paper I) where several N-body barred models of the Galaxy have been built from bar unstable axisymmetric initial conditions, constraining the position of the observer relative to the bar with the COBE K-band data corrected for extinction by dust. The best matching models suggested an angle of [FORMULA] for the bar inclination and a corotation radius of [FORMULA] kpc. Here we take advantage of these simulations to seek the most convenient initial conditions for much larger simulations, including a gas component treated by the smooth particle hydrodynamics (SPH) technique and with no imposed symmetries. The resulting self-consistent gas flows are used to interpret the observed gas kinematics near the Galactic plane, and more especially within the bar region. Contrary to Weiner & Sellwood (1999), our approach is based on the observed bright [FORMULA] features, and not on the mostly low density gas close to the terminal velocities, for which Eulerian codes are more appropriate.

The structure of the paper is organised as follows: in Sect. 2 we describe the main features appearing in the HI and CO [FORMULA] diagrams within the bar region. In Sect. 3 we expose the numerical N-body and SPH code used to perform our simulations. In Sects. 4 and 5, we present the initial conditions and the time evolution of these simulations. In Sect. 6 we select some optimum models from the simulations and interpret the observed gas kinematical features. In Sect. 7 we set some new constraints on the bar parameters based on this interpretation. Finally, Sect. 8 summarises our results. Throughout the paper, we will adopt a solar galactocentric distance [FORMULA] kpc.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999