2013MNRAS.436.2063T

At z < 1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (10⁵-10⁶K). Absorption by the 770.41, 780.32Å doublet of Neviii in quasar spectra represents a unique tool to study this elusive warm-hot phase. We have developed an analytic model for the properties of Neviii absorbers that allows for an inhomogeneous metal distribution. Our model agrees with the predictions of a simulation from the OverWhelmingly Large Simulations project indicating that the average line-of-sight metal-filling fraction within the absorbing gas is low (cL ∼ 0.1). Most of the NevIII in our model is produced in low-density, collisionally ionized gas (nH = 10^–6-10^–4/cm3, T = 10⁵-10⁶K). Strong NevIII absorbers (log10(NNeVIII/cm2)> ?14), like those recently detected by Hubble Space Telescope/Cosmic Origins Spectrograph, are found to arise in higher density gas (nH ≳ 10^–4/cm3, T ~ 5x10⁵K). NevIII cloudlets harbour only 1 percent of the cosmic baryon budget. The baryon content of the surrounding gas (which has similar densities and temperatures as the NevIII cloudlets) is a factor c_L^-1 higher. We conclude that NevIII absorbers are robust probes of shock-heated diffuse gas, but that spectra with signal-to-noise ratios S/N > 100 would be required to detect the bulk of the baryons in warm-hot gas.