Fluctuations of the intergalactic UV background towards two lines of sight ⋆

We present a reanalysis of the He  Lyα absorption towards the quasars HS 1700+6416 and HE 2347-4342 using new high S /N, optical observations. An alternative analysis method is applied, which fits the high quality, optical H  data directly to the He  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. 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  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  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 cutoff 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  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.