Hydrostatic equilibrium in a magnetized, warped Galactic disc

Abstract

Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields treated separately. The regular magnetic field strength required to support the gas is consistent with independent estimates, provided that energy equipartition is maintained between turbulence and random magnetic fields. Our results indicate that a mid-plane value of B0 = 4 μG for the regular magnetic field near the Sun leads to more attractive models than B0 = 2 μG. The vertical profiles of both the regular and random magnetic fields contain disc and halo components, the parameters of which we have determined. The layer at 1≲|z|≲4 kpc can be overpressured and an outflow at a speed of about 50 km s−1 may occur there, presumably associated with a Galactic fountain flow, if B0≃2 μG. We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in z, in rough agreement with H i observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy, and quantify the difference in terms of turbulent velocity and/or magnetic field strength.