Temperature of the IGM/WHIM (Reionisation)
From Eric's Wiki
The intergalactic medium (IGM), a.k.a. the warm-hot intergalactic medium (WHIM) is the source of the absorbtion material.
The temperature-density relation of the gas in cosmological simulations is complicated. Hydrodynamic simulations with shock heating show a tremendous spread in the temperatures (more than 4 orders of magnitidue), particularly in the low-density regions (KRCS04, for eg.). The inclusion of reionisation heating reduces the spread by providing a lower floor to the temperature. With radiative transfer, the situation becomes more complicated with radiative heating dependent on local conditions (BMW04).
A brief history of the gas temperature of the universe from BL04. After recombination, a residual population of free electrons kept the gas close to the CMB temperature (TCMB = 2.725a − 1) until about z∼200. For redshifts of 200 > z > 30, the expanding gas cooled adiabatically (Tad = Toa − 3(γ − 1))) marginally faster than the CMB. It wasn't until the first stars formed that the gas began to heat up, first from the photons emitted between the Lyα and Lyman limit frequencies that would redshift into resonance with the Lyα line, then from the photons with frequencies > the Lyman limit which commenced reionisation.
Presently, the temperature of the IGM is set by a balance between photoionisation heating and adiabatic cooling due to the Hubble expansion. Why not shock heating? In the low-density areas, not much is going on, hence not much shock heating.
Shocks play a role in the heating of the ambient gas. The WHIM (warm-hot intergalactic/cluster medium) is gas with T = 105 − 107K. Within the WHIM is cooler gas, T < 105K. This gas is heated via shocks and reionisation. The situation is reviewed in KRCS04. Shock heating leads to separate temperatures for electrons and ions, due to the slow relaxation time in the ambient medium. Shock heating leads to cooler electrons than ions since shock heating is a kinematic effect and fast-moving ions have higher "temperatures" than fast-moving electrons. Note that this might explain the similar b-parameters of HI and HeII lines which is otherwise explained by bulk kinematic velocities. Indeed, shock heating might "predict" similar b-paramenters. YFH04 point out that since shock-heating leads to a separation of the electron and ion temperatures, simulations of the WHIM must include the separate evolution of the temperatures. They implement such a seaparation into Gadget2. I should look at this paper again to determine the relaxation times between the H and He. YFH04 also show that the spread between electron and ion temperatures is greatest in the hottest regions, whcih makes sense since the hot regions are hot because they are shocked.
--Etittley 11:29, 25 June 2007 (BST)
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