Modelling the spatial distribution of QSO absorption lines

A formalism has been developed for testing specific models for the three-dimensional spatial distributions of Ly-alpha clouds against the line-of-sight observations. The formalism can be used to compute the redshift density of lines and the distribution of line-interval sizes along a line-of-sight for comparison with the observations. A few simple models are considered: a Poisson distribution of Ly-alpha clouds, clouds arising from density fluctuations with a white-noise power spectrum, clouds distributed only on shells of constant comoving radii, clouds restricted to shells that are expanding in a self-similar fashion, and a Poisson distribution of clouds in the 'walls' between small-scale voids.