The Vertical Variation of HI Velocity Dispersion in Disk Galaxies

One of the key assumptions in dynamical applications of the HI velocity dispersion in disk galaxies (e.g. to the flattening of the dark halo) has always been the isothermal nature of the HI distribution. There is no physical reason for this assumption: it is made because until now it has not been possible to measure how the HI velocity dispersion changes with height above the galactic plane. If the velocity dispersion is not isothermal, then the inferred HI pressure gradient is incorrect and the dynamical conclusions will be in error. We have recently published a new technique for measuring the radial and vertical variation of the HI velocity dispersion in edge-on disk galaxies. By carefully observing the HI velocity dispersion in edge-on galaxies ESO 138-G014 and ESO 274-G001 as a function of radius and height above the plane, we will be able for the first time to use the hydrostatics of the HI layer near the plane to probe the shape of the halo and the contribution of the stellar component to the rotation curve.