Fluctuations of the intergalactic UV background towards two lines of sight

Abstract

We present a reanalysis of the HeII Lyman alpha absorption towards the quasars HS1700+6416 and HE2347-4342 using new high S/N, optical observations. An alternative analysis method is applied, which fits the high quality, optical HI data directly to the HeII spectrum. The results are compared to those inferred from standard line profile analyses. This new method enables us to derive redshift scales characterizing the fluctuations of the column density ratio eta. We find eta changing smoothly with redshift on typical scales of Delta z ~ 0.01-0.03 corresponding to 8-24 h^-1 Mpc comoving. The real length scales of variations of the column density ratio might be even larger, since part of the fluctuations may be caused by noise in the HeII data and by effects due to the applied method. However, eta variations on small scales of a few Mpc with an amplitude of about +/- 1.5 dex cannot be ruled out completely. The data shows an apparent correlation between low eta regions and the presence of metal line absorbers, which corresponds to the more general correlation of low eta and strong HI absorption. Thermal line broadening is suggested as a probable explanation for this apparent correlation, since both fit methods would severely underestimate eta for absorbers with log N(HI) > 13 if the line width was dominated by thermal broadening. Indeed, lines located close to the cut-off of the b(N) distribution yield lower column density ratios compared to the whole sample, in particular if high density absorbers are considered. We argue that the apparent correlation of eta with the strength of the HI absorption is caused by insufficient consideration of thermal broadened lines by the standard analysis. As unbiased value of the column density ratio, we find eta ~80 in agreement with previous estimates.

15 pages, 11 figures, recommended for publication in A&A

References in corpus (3)

• The UV spectrum of HS1700+6416 II. FUSE observations of the HeII Lyman alpha forest
• A Unified Fitting of HI and HeII LyαTransmitted Flux of QSO HE2347 with LCDM Hydrodynamic Simulations
• The UV spectrum of HS1700+6416 I. Predicting the metal line content of the far UV spectrum

Cited by in corpus (9)

• The End of Helium Reionization at z~2.7 Inferred from Cosmic Variance in HST/COS HeII Lyman Alpha Absorption Spectra
• Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8 < z < 2.9
• The Implications of Gunn-Peterson Troughs in the HeII Lyman-alpha Forest
• Fluctuations in the Ionizing Background During and After Helium Reionization
• Dating individual quasars with the HeII proximity effect
• Mapping the "invisible" circumgalactic medium around a z $\sim$ 4.5 radio galaxy with MUSE
• The HeII Lyman alpha forest and the thermal state of the IGM
• Spatial Correlations in the Helium-Ionizing Background
• HeII Ly$\alpha$ Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of HeII Reionization