Revisiting the He II to H I ratio in the intergalactic medium

We estimate the He II to H I column density ratio, eta = N(He II)/N(He I), in the intergalactic medium towards the high-redshift (z(em) = 2.885) bright quasar QSO HE 2347-4342 using Voigt-profile fitting of the HI transitions in the Lyman series and the He II Lyman alpha transition as observed by the FUSE satellite. In agreement with previous studies, we find that eta > 50 in most of the Lyman alpha forest, except in four regions where it is much smaller (eta similar to 10-20) and therefore inconsistent with photoionization by the ultraviolet background flux. We detect OVI and C IV absorption lines associated with two of these regions (z(abs) = 2.6346 and 2.6498). We show that, if we constrain the fit of the HI and/or He II absorption profiles with the presence of metal components, we can accommodate eta values in the range 15-100 in these systems, assuming that broadening is intermediate between pure thermal and pure turbulent. While simple photoionization models reproduce the observed N(O VI)/N(C IV) ratio, they fail to produce low eta values, in contrast to models with high temperatures (i.e T >= 10(5) K). The Doppler parameters measured for different species suggest a multiphase nature of the absorbing regions. Therefore, if low eta values were to be confirmed, we would favour a multiphase model in which most of the gas is at a high temperature (> 10(5) K) but the metals, and in particular C IV, are due to a lower-temperature (similar to few 10(4) K) photoionized gas.