Abstract
Absorption lines in front of quasars are the only way to probe cosmic filaments, the intergalactic medium (IGM), and deduce their temperature, characteristic sizes, densities and metallicity (Z). Filaments contain most of the baryons (80 %) that are not found in galaxies. What is their degree of ionization ? It's vital to know how filaments interact with galaxies, how heavy elements are dispersed by galaxies to enrich the IGM, how filaments evolve as a function of z, and what role they play during the reionization epoch. There are several types of absorber : firstly, the Lyman-alpha (Lyα) forest, due to a large number of small clouds on the line of sight, located in the filaments and leaves of the cosmic web ; secondly, when the line of sight crosses a galaxy, there is saturated absorption (DLA) ; and, finally, a Ly-limit system, where all higher-energy photons are absorbed (corresponding to 13.6 eV). For each DLA, we expect the associated CIV or MgII doublets, which are very useful for recognizing the redshifts and abundances of absorbing galaxies. From the 2000s onwards, optical instruments made it possible to observe spectra with high spectral resolution (~ 7 km/s), and the number of absorbers on a line of sight can now be counted in the hundreds. However, the number of absorbers decreases with column density, like a power law, with exponent - 1.6. Observation of absorbers provides an insight into the cosmic evolution of atomic gas. Particularly in the epoch of reionization, the optical thickness rises considerably between z = 5and 7. Comparison with simulations leads to the conclusion that reionization is half-finished at z = 7.5 and completed at z = 5.3, with fairly large HI pockets responsible for the dispersion of the optical thickness. The study of coherence between absorbers could help to identify over-dense regions, and future clusters. However, there are often quasars at the center of these structures, which ionize the entire environment. Absorptions during the reionization epoch can be made with the HI line at 21 cm in metric wavelengths.
