Technique for detecting warm-hot intergalactic gas in quasar UV spectra

The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution shortward of ~ 1200 A. The change of the power-law index from approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being intrinsic to quasars. We consider the possibility that the steepening may result from a tenuous absorption component that is anticorrelated with large mass overdensities. UV sensitive satellites, whose detectors can extend down to 1000 A, can set a useful limit to such an absorption component through the search of a flux increase in the window 1050-1190 A (observer frame) with respect to an extrapolation of the continuum above 1230 A. Since the recent FUSE or HST-STIS data do not show any obvious discontinuity in this region, this effectively rules out the possibility that intergalactic HI absorption is very important, and it is concluded that most if not all of the steepening is intrinsic to quasars. A smaller flux discontinuity of order 1% cannot, however, be ruled out yet and would still be consistent with the warm-hot intergalactic component if it amounts to 30% of the baryonic mass, as predicted by some models of large scale structure formation, provided its temperature lies around 10^{5.5} K.