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

Abstract

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 transition as observed by the FUSE satellite. In agreement with previous studies, we find that η > 50 in most of the Lyman 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≥ 10⁵ Κ). 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 (>105 Κ) but the metals, and in particular C iv, are due to a lower-temperature (∼few 10⁴ Κ) photoionized gas.