Missing Baryons in the Warm - Hot Intergalactic Medium

This review briefly discusses the use of UV absorption lines in the spectra of low-redshift QSOs for the study of the physical conditions, metallicity, and baryonic content of the low−z IGM, with emphasis on implications for the missing baryons problem. Current results on the statistics and baryonic content of intervening, low−z O VI and Lyα absorption-line systems are presented with some comments on overlap between these two classes of absorbers and consequent baryon double-counting problems. From observations of a sample of 16 QSOs observed with the E140M echelle mode of STIS, we find 44 intervening O VI absorbers and 14 associated O VI systems (i.e, systems with zabs ≈ zem). This sample implies that the number of intervening O VI absorbers per unit redshift is dN/dz(O VI) = 23± 4 for lines with rest equivalent width > 30 mÅ. The intervening O VI systems contain at least 7% of the baryons if their typical metallicity is 1/10 solar and the O VI ion fraction is ≤0.2. This finding is consistent with predictions made by cosmological simulations of large-scale structure growth. Recently, a population of remarkably broad Lyα lines have been recognized in low−z quasar spectra. If the breadth of these Lyα features is predominantly due to thermal motions, then these H I absorbers likely harbor an important fraction of the baryons. We present and discuss some examples of the