On the hydrostatic equilibrium of the interstellar gas

Abstract

The hydrostatic equilibrium of the interstellar gas-field system perpendicular to the galactic plane in the neighborhood of the Sun has been reexamined. It is demonstrated that the hydrostatic equilibrium condition of the interstellar gas may be formulated independently from the dynamics of halo gas and halo magnetic fields. The distribution of interstellar gas and magnetic fields satisfying hydrostatic and Poisson's equations with distance above the galactic plane is evaluated and compared with the observed distribution as inferred from 21 cm and synchrotron radiation measurements. It appears from the present study that a single-phase description of the interstellar gas is not adequate to account for the observations, whereas a two-component gas model, assuming that the interstellar gas clouds are not directly coupled to the interstellar magnetic field, seems to provide a natural explanation of the distribution of interstellar gas and magnetic fields perpendicular to the galactic plane up to distances of about 500 pc above and below the midplane.