THE DETECTION OF HIGHLY IONIZED-GAS VIA O-VI ABSORPTION TOWARD QSO HS-1422+2309

We have detected O VI absorption in a Lyman limit system at z = 3.3816 toward QSO HS 1422+2309. We have also detected C IV absorption, but w e have not seen N v, Si IV, or C II. The line widths of O VI and C IV show that the kinetic temperature of the system is 10(4.5) < T < 10(5) K. The system may be collisionally ionized, because the O VI column d ensity is larger than the C IV column density, the temperature is high , and because there may be few or no photons with energies greater tha n 4 ryd, which are needed to photoionize O VI. This is because the Ly alpha forest provides an optical depth of unity in a redshift path of 0.03, less than the typical distance to a bright QSO. If the gas is co llisionally ionized with approximately cosmic abundance ratios, the C IV to O VI ratio gives an ionization temperature of T approximate to 1 0(5.3) K, which is significantly higher than the kinetic temperature m easured by the line widths, perhaps because the gas is out of thermal equilibrium and cooling rapidly, much like the coronal gas of our Gala xy. We have estimated the total hydrogen column density of this system using the observed O VI column together with an assumed metallicity o f -2, and we have found that it is very large: N(H) = 10(20.6) cm(-2) if the system is collisionally ionized, and 10(19.9) cm(-2) if the sys tem is photoionized. If most Lyman limit systems have a similar high-i onization component, and there is some evidence that they do, the mass density in hot and highly ionized gas would be large: Ohm(not) approx imate to 0.4 Ohm(b) (h = 0.7), approximate to 10 times more than in th e cold neutral gas seen in damped Ly alpha systems.