Shock-heated gas in the large-scale structure of the universe

Cosmological shock waves are a ubiquitous consequence of cosmic structure formation. They play a major role in heating baryonic gas in the large-scale structure of the universe. In this contribution we study the shock-heated gas in connection with shocks themselves, using a set of N-body/hydrodynamic simulation data of a LambdaCDM universe. The distributions of shock speed and temperature of shock-heated gas should both reflect the depth of gravitational potential of associated nonlinear structures and thus their morphology. From their distributions we find, in addition to hot gas in and around clusters and groups and warm-hot intergalactic medium (WHIM) with T = 10(5)-10(7) K, mostly in filaments, that there is a significant amount of low-temperature WHIM with T < 10(5) K distributed mostly as sheetlike structures. The low-temperature WHIM was heated and collisionally ionized mainly by shocks with v(sh) less than or similar to 150 km s(-1), while photoionization by the UV and X-ray background radiation is important for metal ions. Not only the WHIM with T = 10(5)-10(7) K but also the WHIM with T < 10(5) K makes up a significant fraction of gas mass, implying that the low-temperature WHIM could be important in resolving the missing-baryon problem. The shock-heated gas in filaments and sheets are manifested best through emission and absorption in the soft X-ray and far-UV bands. We confirm that the WHIM with T = 10(5)-10(7) K makes significant contributions to the soft X-ray background, absorption of highly ionized species such as O VII and O VIII in active galactic nucleus (AGN) spectra, and line emission from O VII and O VIII ions, as pointed out by previous studies. However, the WHIM with T < 10(5) K is the major contributor to the absorption of lower ionized species such as O V and O VI, because these photoionized ions are most abundant in sheets of low density and temperature. On the other hand, lines of O V (630 angstrom) and O VI (1032 angstrom) are emitted mostly from the WHIM with 10(5) < T < 106 K, because they are from collisionally excited ions.