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We estimate the He  ii  to H  i  column density ratio,  η= 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 H  i  transitions in the Lyman series and the He  ii  Lyman α transition as observed by the  FUSE  satellite. In agreement with previous studies, we find that  η &gt; 50  in most of the Lyman α forest, except in four regions where it is much smaller ( η∼ 10–20 ) and therefore inconsistent with photoionization by the ultraviolet background flux. We detect O  vi  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 H  i  and/or He  ii  absorption profiles with the presence of metal components, we can accommodate η 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 η 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 η values were to be confirmed, we would favour a multiphase model in which most of the gas is at a high temperature ( &gt;10 5   K) but the metals, and in particular C  iv , are due to a lower‐temperature (∼few 10 4  K) photoionized gas.

Revisiting the He ii to H i ratio in the intergalactic medium