Large-scale structure in the Lyman-α forest -: II.: Analysis of a group of 10 QSOs

The spatial distribution of Ly alpha forest absorption systems towards 10 QSOs is analysed to search for large-scale structure over the redshift range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are concentrated within a 1-deg(2) field. We employ a technique 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 alpha absorption. We find evidence for large-scale structure in the distribution of Ly alpha forest absorption at the >99 per cent confidence level. In individual spectra we fmd overdense Ly alpha absorption on scales of up to 1200 km s(-1). There is also strong evidence for correlated absorption across line-of-sight pairs separated by <3 h(-1) proper Mpc (q(0) = 0.5). For larger separations the cross-correlation signal becomes progressively less significant.