FAR-ULTRAVIOLET ABSORPTION-SPECTRA OF QUASARS - HOW TO FIND MISSING HOT GAS AND METALS

We show that some high-redshift QSO absorption systems that reveal onl y the H I Lyman series lines at wavelengths visible from the ground ma y be a new class of ultra-high-ionization metal line systems, with met al lines in the far-UV region which is now being explored with satelli tes. At high temperatures or in intense radiation fields metal systems will not show the usual C IV absorption, and O VI will become the mos t prominent metal absorber. At still higher ionization, O VI also beco mes weak, and the strongest metal lines are from Ne VIII, Mg X, and Si XII, which have doublets in the range 500-800 angstrom. Hence very hi gh ionization metal systems will not show metal lines in existing spec tra. Recent X-ray observations show that galaxy halos contain hot gas, so we predict that far-UV spectra of QSOs will also show this gas.