Gas Motions in the Inner Galaxy and the Dynamics of the Galactic Bulge Region

  • O. E. Gerhard
Part of the Reviews in Modern Astronomy book series (MOD.ASTRONOMY, volume 5)


Observations of cold gas in the inner galactic disk show a clumpy, highly asymmetric distribution, with large non-circular velocities and some deviations from the Galactic plane. Recent work is reviewed that contends that the flow of gas in the inner few kpc is dominated by a rapidly rotating bar, with corotation at R ⋍ 2.4 kpc and oriented at an angle of θincl = 16 ± 2° with the line-of-sight to the Galactic Center.

The best evidence in support of this model are (i) the ‘molecular parallelogram’ structure observed in the (l, v) diagram at l ≲ 2°, (ii) the HI terminal velocity curve at l ≳ 2°, and (iii) the molecular ring at R ⋍ 3.5 kpc and the region of low gas densities inwards from there. From the kinematical model the mass distribution and gravitational potential in the inner Galaxy can approximately be determined. Recent direct evidence for a non-axisymmetric shape of the Galactic bulge is discussed in comparison with the predictions of the kinematic model, and it is argued that the rapidly rotating component of the potential is likely to arise from within the inner disk.

Gas falls inwards from x 1 orbits through the bar’s inner Lindblad resonance to x 2 orbits deeper in the potential well. A large fraction of the mass on x 2 orbits is concentrated into the great galactic-centre molecular clouds such as Sgr B, which further lose angular momentum to the bulge by dynamical friction. The resulting average mass inflow is of order 0.1 M/yr, suggesting that the galactic centre is subject to violent events, possibly star-bursts. A significant amount of mass and angular momentum may be added to the inner bulge in this way over a Hubble time.


Gravitational Potential Molecular Cloud Closed Orbit Galactic Center Galactic Plane 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Springer-Verlag Berlin Heidelberg 1992

Authors and Affiliations

  • O. E. Gerhard
    • 1
  1. 1.Landessternwarte, KönigstuhlHeidelbergGermany

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