Journal Article

Revisiting the He <span class="smallCaps">ii</span> to H <span class="smallCaps">i</span> ratio in the intergalactic medium

S. Muzahid, R. Srianand and P. Petitjean

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 4, pages 2193-2202
Published in print February 2011 | ISSN: 0035-8711
Published online January 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17592.x
Revisiting the He ii to H i ratio in the intergalactic medium

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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 (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.

Keywords: intergalactic medium; quasars: absorption lines; quasars: individual: HE 2347−4342

Journal Article.  6908 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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