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

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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 [FORMULA] of the H halo gas is then due to the balance between the turbulent pressure of the HVD component and the gravitational potential [FORMULA]:
c is a constant defined by the vertical scale height, [FORMULA] is the HVD velocity dispersion, and [FORMULA] 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 [FORMULA] cm-2.

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

We modeled the emission of H halo gas corresponding to such a distribution for various scale lengths [FORMULA] and [FORMULA], 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 [FORMULA] kpc and [FORMULA] = 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 [FORMULA].

[FIGURE] Fig. 4. Vertical distribution [FORMULA] of the H halo gas in the solar vicinity as derived from our model calculations.
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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998