Modelling Neutral Hydrogen Discs of Spiral Galaxies

We present an analytical model of a neutral hydrogen disc in a spiral galaxy. The gas disc of the spiral galaxy is assumed to have an exponential surface density profile and be ionized by the cosmic ultraviolet background. To compare with observations, we consider the disc position angle and inclination angle for a sight line going through the galaxy disc. The HI column densities depend on the strength of ionizing field and disk position and inclination. The model was applied to NGC 3198 and the results were compared with observational data. The HI disc profile at large disc radii which can be tested by further HI observations using radio telescopes with larger aperture than present facilities. This HI disc model can be used to predict quasar absorption line systems by galaxy disks if quasar sight lines go through the discs.