A high resolution study of intergalactic O VI absorbers at z~2.3

Abstract

[Abridged] We present a detailed study of the largest sample of intervening O VI systems in the redshift range 1.9 < z < 3.1 detected in high resolution (R ~ 45,000) spectra of 18 bright QSOs observed with VLT/UVES. Based on Voigt profile and apparent optical depth analysis we find that (i) the Doppler parameters of the O VI absorption are usually broader than those of C IV (ii) the column density distribution of O VI is steeper than that of C IV (iii) line spread (\delta v) of the O VI and C IV are strongly correlated (at 5.3\sigma level) with \delta v(O VI) being systematically larger than \delta v(C IV) and (iv) \delta v(O VI) and \delta v(C IV) are also correlated (at > 5\sigma level) with their respective column densities and with N(H I) (3 and 4.5 \sigma respectively). These findings favor the idea that C IV and O VI absorption originate from different phases of a correlated structure and systems with large velocity spread are probably associated with overdense regions. The velocity offset between optical depth weighted redshifts of C IV and O VI absorption is found to be in the range 0 < |\Delta v (O VI - CIV)| < 48 km/s with a median value of 8 km/s. We compare the properties of O VI systems in our sample with that of low redshift (z < 0.5) samples from the literature and find that (i) the O VI components at low-z are systematically wider than at high-z with an enhanced non-thermal contribution to their b-parameter, (ii) the slope of the column density distribution functions for high and low-z are consistent, (iii) range in gas temperature estimated from a subsample of well aligned absorbers are similar at both high and low-z, and (iv) \Omega_{O VI} = (1.0 \pm 0.2) \times10^{-7} for N(O VI) > 10^{13.7} cm^{-2}, estimated in our high-z sample, is very similar to low-z estimations.