The Detection of Highly Ionized Gas via O [CSC]vi[/CSC] Absorption Toward QSO HS [FORMULA][F]1422+2309[/F][/FORMULA

Abstract

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 we 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 104.5 < T < 105 K. The system may be collisionally ionized, because the O VI column density is larger than the C IV column density, the temperature is high, and because there may be few or no photons with energies greater than 4 ryd, which are needed to photoionize O VI. This is because the Lyα 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 collisionally ionized with approximately cosmic abundance ratios, the C IV to O VI ratio gives an ionization temperature of T ≈ 105.3 K, which is significantly higher than the kinetic temperature measured by the line widths, perhaps because the gas is out of thermal equilibrium and cooling rapidly, much like the coronal gas of our Galaxy. We have estimated the total hydrogen column density of this system using the observed O VI column together with an assumed metallicity of -2, and we have found that it is very large: N(H) = 1020.6 cm-2 if the system is collisionally ionized, and 1019.9 cm-2 if the system is photoionized. If most Lyman limit systems have a similar high-ionization component, and there is some evidence that they do, the mass density in hot and highly ionized gas would be large: Ωhot ≈ 0.4Ωb (h=0.7), ≈ 10 times more than in the cold neutral gas seen in damped Lyα systems.