As recent cosmological simulations imply, the temperature of the intergalactic medium (IGM) undergoes a significant change from high to low redshifts parallel to the proceeding of large-scale structure formation in the Universe (e.g., Cen & Ostriker 1999; Davé et al. 2001). As a result, a substantial fraction of the baryonic matter in the local Universe is expected to reside in the so-called Warm-Hot Intergalactic Medium (WHIM). The WHIM represents a low-density (nH ~ 10-6 - 10-4 cm-3), high-temperature (T ~ 105 - 107 K) plasma that primarily is made of protons, electrons, He II, and He III, together with traces of some highly-ionised heavy elements. The WHIM is believed to emerge from intergalactic gas that is shock-heated to high temperatures as the medium is collapsing under the action of gravity in large-scale filaments (e.g., Valageas et al. 2002). In this scenario, part of the warm (photoionised) intergalactic medium that gives rise to the Ly forest in the spectra of distant quasars (QSO) is falling in to the potential wells of the increasingly pronounced filaments, gains energy (through gravity), and is heated to high temperatures by shocks that run through the plasma.
Because of the low density and the high degree of ionisation, direct observations of the shock-heated and collisionally ionised WHIM are challenging with current instrumentation (in contrast to the photoionised IGM, which is easily observable through the Ly forest). Diffuse emission from the WHIM plasma must have a very low surface brightness and its detection awaits UV and X-ray observatories more sensitive than currently available (see, e.g., Fang et al. 2005; Kawahara et al. 2006). The most promising approach to study the WHIM with observations at low redshift is to search for absorption features from the WHIM in FUV and in the X-ray regime in the spectra of quasars, active galactic nuclei (AGN) and other suited extragalactic background sources. As the WHIM represents a highly-ionised plasma, the most important WHIM absorption lines are those originating from the electronic transitions of high-ionisation state ions (hereafter referred to as "high ions") of abundant heavy elements such as oxygen and carbon. Among these, five-times ionised oxygen (O VI) is the most valuable high ion to trace the WHIM at temperatures of T ~ 3 × 105 K in the FUV regime. In the X-ray band, the O VII and O VIII transitions represent the key observables to trace the WHIM at higher temperatures in the range 3 × 105 < T < 107. In addition to the spectral signatures of high ions of heavy elements the search for broad and shallow Ly absorption from the tiny fraction of neutral hydrogen in the WHIM represents another possibility to identify and study the most massive WHIM filaments in the intergalactic medium with FUV absorption spectroscopy. Finally, for the interpretation of the observed WHIM absorption features in UV and X-ray spectra the comparison between real data and artificial spectra generated by numerical simulations that include realistic gas physics is of great importance to identify possible pitfalls related to technical and physical issues such as limited signal-to-noise ratios and spectral resolution, line-broadening mechanisms, non-equilibrium conditions, and others.
In this chapter, we review the physics and methodology of the UV and X-ray absorption measurements of warm-hot intergalactic gas at low redshift and summarise the results of recent observations obtained with space-based observatories. The outline of this chapter is the following. The ionisation conditions of the WHIM and the most important absorption signatures of this gas in the UV and X-ray band are presented in Sect. 2. Recent UV absorption measurements of the WHIM at low redshift are discussed in Sect. 3. Similarly, measurements of the WHIM in the X-ray are presented in Sect. 4. In Sect. 5 we compare the results from WHIM observations with predictions from numerical simulations and give an overview of WHIM measurements at high redshift. Finally, some concluding remarks are given in Sect. 5.