Abstract
We study the temperature structure of the intergalactic medium (IGM) using a large cosmological N-body/smoothed particle hydrodynamics simulation. We employ a two-temperature model for the thermal evolution of the ionized gas, in which we include explicitly the relaxation process between electrons and ions. In the diffuse, hot IGM, the relaxation time is comparable to the age of the universe, and hence the electron temperature in postshock regions remains significantly smaller than the ion temperature. We show that, at the present epoch, a large fraction of the warm-hot intergalactic medium (WHIM) has a well-developed two-temperature structure, with typical temperature differences of order a few. Consequently, the fraction of metals in various ionization states such as O VI, O VII, and O VIII, as well as their line emissivities, can differ locally by more than an order of magnitude from those computed with a single-temperature model, especially in gas with T ~ 107 K. It is thus necessary to follow the evolution of the electron temperature explicitly to determine absorption and emission by the WHIM. Although equipartition is nearly achieved in the denser intracluster medium, we find an appreciable systematic deviation between the gas mass weighted electron temperature and the mean temperature even at half the virial radii of clusters. There is thus a reservoir of warm (Te < 1 keV) gas in and around massive clusters. Our results imply that relaxation processes need to be considered in describing and interpreting observational data from existing X-ray telescopes as well as from future missions designed to detect the WHIM, such as the Diffuse Intergalactic Oxygen Surveyor and the Missing Baryon Explorer.
Highlights
The distribution of baryons in the Universe remains one of the puzzling issues in modern cosmology
The total amount of baryons inferred from a census of Hi absorption, gas and stars in galaxies, and X-ray emission from hot gas in galaxy clusters is far smaller than that predicted by nucleosynthesis calculations (Persic & Salucci 1992; Fukugita, Hogan & Peebles 1998) and by measurements of the cosmic microwave background radiation (Spergel et al 2003)
Three dimensional hydrodynamic simulations of cosmic structure formation (e.g., Cen & Ostriker 1999; Dave et al 2001; Croft et al 2002) predict that about 30–40% of the baryons reside in the so-called warm-hot intergalactic medium (WHIM) at the present epoch
Summary
The distribution of baryons in the Universe remains one of the puzzling issues in modern cosmology. This gas is mostly shock-heated to a temperature of ∼ 105–107 K during large scale structure formation, and this relatively low temperature makes its thermal emission difficult to detect with conventional X-ray probes.
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