5 Conclusions

Our results on O VI absorbing clouds in HE 0515-4414 can be summarized as follows:

• Intervening O VI systems per unit redshift appear to be less frequent by a factor of $\sim$5 at $\bar{z} = 1.5$ compared to the local universe $\bar{z} = 0.21$.
• According to the observed velocities, the O VI lines are not formed in the same volume as the H I and C IV absorbing material. This conclusion is supported by the derived Doppler parameters (see Table 1). An identical gas phase would require $b_{\rm O{\sc vi}} \le b_{\rm C{\sc iv}}$ in contradiction with the observations.
• Consequently, even narrow related C IV lines which would eliminate collisional ionization as mechanism for C IV production, cannot rule out collisional ionization for O VI from observations. For the same reason, O VI/C IV column density ratios cannot be used for arguing in favour of or against photoionization/collisional ionization.

The occurrence of an extremely broad component superimposed on the "normal" Doppler profile as observed in the z = 1.674 system is a rare Ly $\alpha$ profile type. In fact, we have never seen such a profile combination. In the context of modern interpretations of the Ly $\alpha$ forest as caused by a gradually varying density field characterized by a network of filaments and sheets (e.g. Bi & Davidsen 1997), a multi-component Voigt profile fitting is artificial and without a physical meaning anyway. The observed Ly $\alpha$ profile at z = 1.674 could be easily modelled by an overdense structure with inflow or outflow velocities of the order of $100\,{\rm km\,s^{-1}}$. We abstain from such an exercise, since the line profile decomposition would not lead to a unique solution. The coincidence with a strong O VI doublet at the same velocity is remarkable.

Our finding, that the O VI phase at $\bar{z} = 1.5$contains a factor of $\geq 10$ less material than at z = 0.21, provided the O VI/O ratio and the oxygen abundance are similar, appears to be inconsistent with the simulations of Davé et al. (2001) who predict an increase of the mass-fraction of baryons in the warm-hot phase of the IGM by at most a factor of 4 between z = 1.5 and 0.2. An increase in the mean oxygen abundance in the low density IGM by a factor of $\sim$3 over the same redshift range would restore consistency with the theoretical predictions. However, at present we do not see a possibility to test this hypothesis. Furthermore, as long as we do not understand the ionization to O VI quantitatively, the fractional ionization O VI/O might vary between z = 1.5 and 0.2. Finally, our result is still debatable due to small number statistics. More lines of sight, both at low and intermediate redshift, have to be probed.

Acknowledgements

This work has been supported by the Verbundforschung of the BMBF/DLR under Grant No. 50 OR 99111. S.L. acknowledges financial support by FONDECYT grant N $^{\rm o} 3\,000\,001$ and by the Deutsche Zentralstelle für Arbeitsvermittlung.