Revisiting the He II to H I ratio in the intergalactic medium
Résumé
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 (zem= 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 > 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 (zabs= 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 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 (>105 K) but the metals, and in particular C IV, are due to a lower-temperature (∼few 104 K) photoionized gas.
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