CLEAR EVIDENCE FOR THE PRESENCE OF O-VI ABSORPTION IN QSO METAL SYSTEMS

We have detected O VI lambdalambda1031.93, 1037.62 doublet absorption in a composite QSO spectrum formed from a large number of intervening C IV absorption systems. Numerical simulations show that the detections are highly significant. The detections constitute the first firm evidence for the presence of O VI in intervening QSO metal absorption systems. The equivalent width of the detected O VI absorption implies an O VI column density N(O VI) greater-than-or-equal-to 3.8 x 10(14) cm-2. This value, together with the nondetection of the N V lambdalambda1238.82, 1242.80 doublet absorption, suggests that N(O VI)/N(N V) greater-than-or-equal-to 4.4. For collisionally ionized gas with a solar 0 to N abundance ratio in thermal equilibrium the above ratio requires a temperature T greater-than-or-equal-to 2.5 x 10(5) K. It is found that C IV systems which show low-ionization species and those which do not both have associated O VI absorption, suggesting that O VI is probably present in all C IV systems. We also find that C IV systems which show low-ionization species on average have stronger high-ionization absorption lines than those which do not. A simple interpretation was given to explain this trend. The possibility of using O VI absorption as a discriminator of different ionization mechanisms of the absorbing gas is discussed.