Vertical Equilibrium of HI in the Galactic Disk

Extract

The analysis of vertical distribution and kinematics of disks of galaxies can yield the local mass density in the disk, distinct from the integrated mass inside a radius, derived from a rotation curve. HI is a particularly good tracer because of its ubiquity, ease of observation, and near isothermal nature. In the simplest case one considers the turbulent pressure gradient of the gas balancing the gravitational force in the z-direction. But the atomic gas may be subject to other pressures, for example, magnetic, cosmic ray or radiation pressure. The relative contributions of these are essentially unconstrained. The mass densities obtained from the analysis of HI vertical equilibrium can be verified with a similar analysis only in the solar neighborhood, where the vertical distribution and kinematics of stars can give an independent measure of the total midplane mass density (ρ₀).