High-resolution spectroscopy of the 3D cosmic web with close QSO groups

We study the 3D distribution of matter at z similar to 2 using high-resolution spectra of quasi-stellar object (QSO) pairs and simulated spectra drawn from cosmological hydrodynamical simulations. We present a sample of 15 QSOs, corresponding to 21 baselines of angular separations evenly distributed between similar to 1 and 14 arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory Very Large Telescope (ESO-VLT). The observed correlation functions of the transmitted flux in the H i Lyman alpha forest transverse to and along the line of sight are in agreement, implying that the distortions in redshift space due to peculiar velocities are relatively small and - within the relatively large error bars - not significant. The clustering signal is significant up to velocity separations of similar to 300 km s-1, corresponding to about 5 h-1 comoving Mpc. Compatibility at the 2 Sigma level has been found both for the autocorrelation and cross-correlation functions and for the set of the cross-correlation coefficients. The analysis focuses in particular on two QSO groups of the sample, the Sextet and the Triplet. Searching for alignments in the redshift space between Lyman alpha absorption lines belonging to different lines of sight, it has been possible to discover the presence of a wide H i structure extending over about 10 Mpc in comoving space, and give constraints on the sizes of two cosmic underdense regions in the intergalactic medium, which have been detected with a 91 and 86 per cent significance level, respectively, in the Sextet and the Triplet.