Evidence of Structure in the Lyman- α Forest

The spatial distribution of Ly-α forest absorption systems toward a group of eight closely spaced QSOs has been analysed and evidence for large scale structure has been found at < z > = 2.8. Our technique is based on the first and second moments of the transmission probability density function which is capable of identifying and assessing the significance of regions of over- or underdense Ly-α absorption. The data has revealed at least two interesting features. 1. An overdense structure at z = 2.27 which extends at least over ~8 h^-1 comoving Mpc (q₀ = 0.5) in the plane of the sky. Metal absorption lines have been found at the same redshift and thus a cluster or proto-cluster of galaxies seems to have been discovered. 2. A void at z = 2.97, extending over ~20 h^-1 comoving Mpc in the plane of the sky, possibly caused by a locally increased UV ionising flux due to a foreground QSO.