MODELING THE SPATIAL-DISTRIBUTION OF QSO ABSORPTION-LINES

The magnitude and the scale of clustering of Ly alpha-absorption clouds provide potentially important clues regarding their origin and spatial distribution. Such clues may be useful in distinguishing between different theories for the formation of large-scale structure; theories such as the cold dark matter model and the explosion scenario predict different spatial distributions for the clouds. Hence, we have developed a formalism for testing specific models for the three-dimensional spatial distributions of Ly alpha clouds against the line-of-sight observations. We use the formalism to compute the redshift density of lines and the distribution of line-interval sizes along a line-of-sight for comparison with the observations. We consider a few simple models: 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. The Poisson distribution without voids and the voided Poisson model with approximately 3.5 h0(-1) Mpc voids cannot account for the observed correlations. On the other hand, these correlations are too weak to support the shell hypothesis where the shells are isolated. The graviational clustering model is consistent with the observations and the associated Ly alpha-cloud correlation length, extrapolated to the present, is approximately 0.43 h0(-1) Mpc, approximately a factor of 10 smaller than that for the galaxies.