Structure in the Lyα Forest

The spatial distribution of Lyα forest absorption systems toward a group of 8, 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 Lyα 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.