Astron. Astrophys. 327, 983-1003 (1997)

3D self-consistent N -body barred models of the Milky Way

I. Stellar dynamics

R. Fux

Geneva Observatory, Ch. des Maillettes 51, CH-1290 Sauverny, Switzerland

Received 23 April 1997 / Accepted 20 June 1997

Abstract

Many 3D N -body barred models of the Galaxy extending beyond the Solar circle are realised by self-consistent evolution of various bar unstable axisymmetric models. The COBE/DIRBE K -band map, corrected for extinction, is used to constrain the location of the observer in these models, assuming a constant mass-to-light ratio. The resulting view points in the best matching models suggest that the inclination angle of the Galactic bar relative to the Sun-Galactic centre line is .

Scaling the masses according to the observed radial velocity dispersion of M giants in Baade's Window, several models reproduce satisfactorily the kinematics of disc and halo stars in the Solar neighbourhood, as well as the disc local surface density and scale parameters. These models have a face-on bar axis ratio and a bar pattern speed  km/s/kpc, corresponding to a corotation radius of  kpc. The HI terminal velocity constraints favour models with low disc mass fraction near the centre.

The large microlensing optical depths observed towards the Galactic bulge exclude models with a disc scale height  pc around  kpc, arguing for a constant thickness Galactic disc. The models also indicate that a spiral arm starting at the near end of the bar can contribute as much as to the optical depth in Baade's Window. The mass-to-K luminosity ratio of the Galactic bulge is probably more than 0.7 (Solar units), and if the same ratio applies outside the bar region, then the Milky Way should have a maximum disc.

Key words: Galaxy: structure; kinematics and dynamics

Send offprint requests to: R. Fux

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Contents

• 1. Introduction
• 2. Initial conditions
  • 2.1. Mass distribution
  • 2.2. Truncation and flattening
  • 2.3. Choice of mass density parameters
  • 2.4. Isotropic Gaussian velocity distribution
  • 2.5. Anisotropic velocity dispersion
  • 2.6. Dark halo disequilibrium and velocity distribution
  • 2.7. Choice of velocity parameters
• 3. Time evolution
  • 3.1. N -body code
  • 3.2. Model evolution
• 4. Location of the observer
  • 4.1. Reduction of the COBE K -band map
  • 4.2. Fitting method
  • 4.3. Tests
  • 4.4. Application
• 5. Model properties and discussion
  • 5.1. Local properties
  • 5.2. Corotation and terminal velocities from orbits
  • 5.3. Mass-to-K luminosity ratio
  • 5.4. Bulge microlensing
  • 5.5. Inner kinematics
• 6. Conclusion
• Acknowledgements
• Appendix
  • Appendix A: a non-Gaussian bounded 3D distribution
  • Appendix B: the residual
  • Appendix C: the variance of the model fluxes
• References

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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