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 to be ionized by the cosmic ultraviolet background. To compare with observations, we consider the disc position angle and inclination angle for a line of sight going through the galaxy disc. The HI column densities depend on the strength of ionizing field and disc 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 can be tested by further HI observations using radio telescopes with a larger aperture than the present facilities. This HI disc model can be used to predict quasar absorption line systems by galaxy discs if quasar lines of sight go through the discs