Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs

We study the three-dimensional distribution of non-virialized 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 have collected the largest sample of QSO pairs ever observed with Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory-Very Large Telescope (ESO-VLT), with angular separations between similar to 1 and 14 arcmin. The observed correlation functions of the transmitted flux in the H I Lyman alpha forest along and transverse to the lines of sight are in good agreement implying that the distortions in redshift space due to peculiar velocities are small. The clustering signal is significant up to velocity separations of similar to 200 km s(-1), or about 3 h(-1) comoving Mpc. The regions at lower overdensity (rho/(rho) over bar less than or similar to 6.5) are still clustered but on smaller scales (Delta v less than or similar to 100 km s(-1)). The observed and simulated correlation functions are compatible at the 3 sigma level. A better concordance is obtained when only the low overdensity regions are selected for the analysis or when the effective optical depth of the simulated spectra is increased artificially, suggesting a deficiency of strong lines in the simulated spectra. We found that also a lower value of the power-law index of the temperature-density relation for the Lyman a forest gas improves the agreement between observed and simulated results. If confirmed, this would be consistent with other observations favouring a late He II reionization epoch (at z similar to 3). We remark the detection of a significant clustering signal in the cross-correlation coefficient at a transverse velocity separation Delta v perpendicular to similar to 500 km s(-1) whose origin needs further investigation.