Absorption signatures of warm-hot gas at low redshift: Ne viii

At z {lt} 1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (10$^{5}$-10$^{6}$ K). Absorption by the 770.41, 780.32 a doublet of Ne VIII in quasar spectra represents a unique tool to study this elusive warm-hot phase. We have developed an analytic model for the properties of Ne VIII 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 (c$_L$ { ilde} 0.1). Most of the Ne VIII in our model is produced in low-density, collisionally ionized gas (n$_H$ = 10$^{-6}$-10$^{-4}$ cm$^{-3}$, T = 10$^{5}$-10$^{6}$ K). Strong Ne VIII absorbers (log$_{10}$(N$_{NeVIII}$/cm$^{-2}$){gsim}14), like those recently detected by Hubble Space Telescope/Cosmic Origins Spectrograph, are found to arise in higher density gas (n$_H$ {gsim} 10$^{-4}$ cm$^{-3}$, T {ap} 5 { imes} 10$^{5}$ K). Ne VIII cloudlets harbour only 1 per cent of the cosmic baryon budget. The baryon content of the surrounding gas (which has similar densities and temperatures as the Ne VIII cloudlets) is a factor c_L^{}${$-1$}$ higher. We conclude that Ne VIII absorbers are robust probes of shock-heated diffuse gas, but that spectra with signal-to-noise ratios S/N {gt} 100 would be required to detect the bulk of the baryons in warm-hot gas.