Astron. Astrophys. 339, 745-758 (1998)

Hydrostatic equilibrium conditions in the galactic halo

P.M.W. Kalberla and J. Kerp

Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany

Received 19 March 1998 / Accepted 25 August 1998

Abstract

The large scale distributions of gas, magnetic field and cosmic rays in the galactic halo are investigated. Our model is based on the analysis of all-sky surveys of H I  gas (Leiden/Dwingeloo survey), soft X-ray radiation (ROSAT all-sky survey), high energy -ray emission (EGRET 100 MeV), and radio-continuum emission (408 MHz survey).

We found a stable hydrostatic equilibrium configuration of the Galaxy which, on large scales, is consistent with the observations. Instabilities due to local pressure or temperature fluctuations can evolve only beyond a scale height of 4 kpc. We have to distinguish 3 domains, with different physical properties and scale heights:

1) The gaseous halo has an exponential scale height 4.4 kpc. Its radial distribution is characterised by a galactocentric scale length kpc. On large scales all components of the halo - gas, magnetic fields and cosmic rays - are in pressure equilibrium. The global magnetic field is regularly ordered and oriented parallel to the galactic plane.

2) The disk has a vertical scale height of about 0.4 kpc. Characteristic for this region is the high gas pressure. The associated magnetic field is irregularly ordered and its equivalent pressure is only 1/3 of the gas pressure. The cosmic rays are decoupled from gas and magnetic fields.

3) The diffuse ionised gas layer with a vertical scale height of about 0.95 kpc and a radial scale length of kpc acts as a disk-halo interface. The magnetic field in this region has properties similar to that in the disk. However, here the cosmic rays are coupled to the magnetic fields in contrast to the situation within the galactic disk. The gas pressure in this transition region is essential for the stability of the galactic halo system.

Applying the model we can derive some major properties of the Milky Way:

Assuming that the distribution of the gas in the halo traces the dark matter, we derive for a flat rotation curve a total mass of . The mass of the galactic halo is .

We find that turbulent motions in the gaseous halo can be described by the Kolmogoroff relation. The smallest clouds, which are compatible with such a turbulent flow, are at temperatures close to 3 K. They have linear sizes of 20 au and masses of . A significant fraction of the galactic dark matter may be in this form.

Key words: Galaxy: halo Galaxy: kinematics and dynamics ISM: clouds cosmic rays ISM: magnetic fields dark matter

Send offprint requests to: P. Kalberla, (pkalberla@astro.uni-bonn.de)

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Contents

• 1. Introduction
• 2. Equilibrium conditions
  • 2.1. Basics
  • 2.2. Gravity
• 3. Data
  • 3.1. The composition of the galactic gas phase
  • 3.2. The galactic halo
    • 3.2.1. Neutral gas in the halo
    • 3.2.2. X-ray gas in the halo
    • 3.2.3. Highly ionised gas in the halo
  • 3.3. The disk-halo interface
  • 3.4. The galactic disk
  • 3.5. Magnetic field and cosmic rays
• 4. The model
  • 4.1. Radial distribution
  • 4.2. Vertical distribution
  • 4.3. Galactic rotation
• 5. Verification of the model
  • 5.1. Synchrotron radiation
    • 5.1.1. The galactic halo
    • 5.1.2. The disk-halo interface
    • 5.1.3. The galactic disk
  • 5.2. -rays
  • 5.3. Scaling relations
• 6. Stability considerations
• 7. The gaseous halo - a tracer of dark matter?
• 8. Turbulence
• 9. Summary and discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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