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(H 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-$\alpha$ 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 ($\eta \sim 10-20$) and therefore inconsistent with photo-ionization by the UV background flux. We detect O VI and C IV absorption lines associated with two of these regions ($z_{\mathrm{abs}}$ = 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 $\eta$ values in the range 15-100 in these systems assuming broadening is intermediate between pure thermal and pure turbulent. While simple photo-ionization models reproduce the observed N(O VI)/N(C IV) ratio, they fail to produce low $\eta$ values contrary to models with high temperature (i.e T $\ge 10^5$ 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 favor a multi-phase model in which most of the gas is at high temperature ($>$ 10${}^5$ K) but the metals and in particular C IV are due to lower temperature ($\sim$ few $10^4$ K) photo-ionized gas.

Accepted for publication in the MNRAS (11 pages, 9 figures, 2 tables)

References in corpus (7)

• Time-Dependent Ionization in Radiatively Cooling Gas
• A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers
• The Nature and Origin of Low-Redshift O VI Absorbers
• Metallicity of the intergalactic medium using pixel statistics: IV. Oxygen
• The missing metals problem. III How many metals are expelled from galaxies?
• Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
• Fluctuations of the intergalactic UV background towards two lines of sight

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• A high resolution study of intergalactic O VI absorbers at z~2.3
• Probing the large and massive CGM of a galaxy at z ~ 0.2 using a pair of quasars
• The He II Post-Reionization Epoch: HST/COS Observations of the Quasar HS1700+6416
• The Effect of Fluctuations on the Helium-Ionizing Background
• The case against large intensity fluctuations in the z ~ 2.5 HeII Lyman-alpha forest
• The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon
• He II optical depth and UV escape fraction of galaxies
• Probing Inhomogeneity in the Helium Ionizing UV Background