Astron. Astrophys. 332, L61-L64 (1998)

4. Hydrostatic equilibrium model

We interpret the low-intensity profile wings at the extreme velocities as emission from neutral gas in hydrostatic equilibrium with the gravitational potential of the Galaxy. The density distribution of the H I  halo gas is then due to the balance between the turbulent pressure of the HVD component and the gravitational potential :

c is a constant defined by the vertical scale height, is the HVD velocity dispersion, and is the gravitational potential as given by Kuijken & Gilmore (1989). From the column density of the HVD component in direction to the north galactic pole we obtain the constraint cm^-2.

We model the distribution of the halo gas throughout the Galaxy following the approach of Taylor & Cordes (1993), separating radial and horizontal dependencies: where is the mid-plane density and defines the radial density distribution; = 8.5 kpc.

We modeled the emission of H I  halo gas corresponding to such a distribution for various scale lengths and , assuming that the halo gas is co-rotating with the disk. The rotation curve was taken from Fich et al. (1990). The best fit to the observations is given in Fig. 1 for the scale lengths kpc and = 15 kpc. This result yields a value of c =3, implying a halo model where gas, magnetic fields and cosmic rays are in pressure equilibrium. Fig. 4 shows the corresponding distribution .

Fig. 4. Vertical distribution of the H I  halo gas in the solar vicinity as derived from our model calculations.

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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