The intergalactic medium
The intergalactic medium (IGM), diffuse, gaseous material that occupies the space between the galaxies, has become a fundamental observable of cosmology in the last ten years, thanks both to high resolution spectroscopy and theoretical progress that have allowed the modeling of the relevant physical processes. By observing the spectra of very high redshift quasars (z>5), it is possible to measure the ionisation state of the intergalactic medium at epochs close to cosmic re-ionisation. In the same way the primordial metal enrichment can be estimated and its link to putative population III stars investigated. The presence of metals favours the cooling and fragmentation of pre-stellar clouds, making the formation of stars possible when the metallicity of the gas reaches a critical value. The so-called Lyman- absorption line forest, produced by neutral hydrogen along the line of sight, also allows the measurement of the density of the material and the clustering of the absorbing clouds, at epochs where it is not possible with the galaxies. Models show that at low redshift a significant fraction (about 50%) of "ordinary" (so-called baryonic) matter remains trapped in a "warm" phase within the filamentary dark matter structures that connect the halos in which galaxy clusters form. The Italian community is taking a centre-stage role in these studies, with analysis of the observations and development of hydrodynamic codes for the simulation of cosmic structures and the IGM that incorporate the most relevant physical processes (cooling, heating by the ultraviolet background). In this area, the Italian community actively participates in the development of new generation, high resolution spectrographs for ground-based telescopes. After having built the X-Shooter spectrograph for one of the 8-metre VLT telescopes, ESPRESSO is now in the construction phase; this will simultaneously use the light from all four telescopes. ESPRESSO represents the precursor and test-bench for CODEX, one of the first generation instruments for the future European Extremely Large Telescope (EELT) that will aim to directly measure the differential expansion of the Universe by measuring the position of spectral lines with very high precision. For space-based research into the missing baryons, instead, participation in IXO remains important, the X-ray astronomy satellite developed by an ESA-NASA-JAXA collaboration.
