Fluctuations of the intergalactic UV background towards two lines of sight

Abstract

Aims. We present a reanalysis of the He ii Lyα absorption towards the quasars HS 1700+6416 and HE 2347-4342 using new high S /N, optical observations.Methods. An alternative analysis method is applied, which fits the high quality, optical H i data directly to the He ii 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 η. Results. We find η changing smoothly with redshift on typical scales of ∆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 He ii data and by systematic effects due to the applied method. However, variations on small scales cannot be ruled out completely. For η variations on scales of a few Mpc an amplitude of about ±1.5 dex cannot be excluded. The data shows an apparent correlation between low η regions and the presence of metal line absorbers, which corresponds to the more general correlation of low η and strong H i absorption. Thermal line broadening is suggested as a probable explanation for this apparent correlation, since both fit methods would severely underestimate η for absorbers with N H i > ∼ 10 13 cm −2 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 η with the strength of the H i absorption is caused by insufficient consideration of thermal broadened lines by the standard analysis. As unbiased value of the column density ratio, we find η ∼ 80 in agreement with previous estimates.

CONSTRAINTS ON QUASAR LIFETIMES AND BEAMING FROM THE He II Lyα FOREST

“…In that case, η ∼ 40-80 Zheng et al 2004;Fechner et al 2006;Fechner & Reimers 2007) implies r 0 ∼ 110-160 Mpc at z ∼ 2.5. A separate argument comes from matching the (measured) emissivity of the quasar population with the observed optical depth of the He ii forest.…”

“…The best sample so far surrounds the line of sight to HE 2347-4342, whose He ii forest has been extensively studied Shull et al 2004;Fechner & Reimers 2007;Shull et al 2010). Worseck et al (2007) searched for nearby quasars with L min 1-2 × 10 11 L and a maximum impact parameter ∼12 comoving Mpc.…”

The HeII Lyman alpha forest and the thermal state of the IGM

“…Three different analysis methods have been applied to the observations of the He  Lyα forest so far: line profile fitting (Kriss et al 2001;Zheng et al 2004;Fechner et al 2006a), an apparent optical depth method (Shull et al 2004;Fechner et al 2006a) and spectrum fitting (Fechner & Reimers 2007). The latter fits the high-quality optical H  spectrum directly to the He  data, and allows to estimate the length scales of η variations.…”

“…There are several suggestions for a physical explanation of this correlation like self-shielding and shadowing effects (Maselli & Ferrara 2005;Tittley & Meiksin 2006) and the spatial distribution of QSOs as the sources of hard radiation (Bolton et al 2006). However, Fechner & Reimers (2007) suggested that part of the correlation might be due to the unjustified assumption that the line widths of the absorption features are dominated by purely turbulent broadening. If this assumption is incorrect, a correlation between the η value and the absorber density will be introduced artificially.…”

“…Liu et al (2006) claimed to confirm this finding using hydrodynamic simulations. On the other hand, Fechner & Reimers (2007) showed that statistically, lines with apparent low η values are often dominated by thermal broadening. In order to solve this apparent conflict further investigation is needed.…”