Astron. Astrophys. 326, 554-564 (1997)

Vertical equilibrium of molecular gas in galaxies

F. Combes and J-F. Becquaert

Observatoire de Paris, DEMIRM, 61 av. de l'Observatoire, F-75014 Paris, France

Received 30 January 1997 / Accepted 22 May 1997

Abstract

We present CO(1-0) and CO(2-1) observations of the two nearly face-on galaxies NGC 628 and NGC 3938, in particular cuts along the major and minor axis. The contribution of the beam-smeared in-plane velocity gradients to the observed velocity width is quite small in the outer parts of the galaxies. This allows us to derive the velocity disper- sion of the molecular gas perpendicular to the plane. We find that this dispersion is remarkably constant with radius, 6 for NGC 628 and 8.5 for NGC 3938, and of the same order as the H I dispersion. The constancy of the value is interpreted in terms of a feedback mechanism involving gravitational instabilities and gas dissipation. The similarity of the CO and H I dispersions suggests that the two components are well mixed, and are only two different phases of the same kinematical gas component. The gas can be transformed from the atomic phase to the molecular phase and vice-versa several times during a z-oscillation.

Key words: interstellar medium: kinematics and dynamics galaxies: individual: ngc 628 galaxies: individual: ngc 3938 galaxies: ism galaxies: kinematics and dynamics galaxies: spiral

Send offprint requests to: F. Combes

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Contents

• 1. Introduction
• 2. Relevant galaxy properties
  • 2.1. NGC 628
  • 2.2. NGC 3938
• 3. Observations
• 4. Results
  • 4.1. Spectra and radial distributions
  • 4.2. Vertical velocity dispersion
  • 4.3. Comparison of the two galaxies
• 5. Summary and Discussion
  • 5.1. Critical dispersion for gravitational stability
  • 5.2. Similar H I and CO vertical dispersions
    • 5.2.1. Two phases of the same dynamical component
    • 5.2.2. Collisions
    • 5.2.3. Gravitational heating
    • 5.2.4. H I and CO velocity dispersions in the Milky Way
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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