Probing inhomogeneity in the helium ionizing UV background

Abstract

We present an analysis combining the simultaneous measurement of intergalactic absorption by hydrogen ({{H {I}}}), helium ({{He {II}}}), and oxygen ({{O {VI}}}) in UV and optical quasar spectra. The combination of the {{H {I}}} and {{He {II}}} Lyman-alpha forests through η (the ratio of column densities of singly ionized helium to neutral hydrogen) is thought to be sensitive to large-scale inhomogeneities in the extragalactic UV background. We test this assertion by measuring associated five-times-ionized oxygen ({{O {VI}}}) absorption, which is also sensitive to the UV background. We apply the pixel optical depth technique to {{O {VI}}} absorption in high and low η samples filtered on various scales. This filtering scale is intended to represent the scale of any coherent oxygen excess/deficit. We find a 2σ detection of an {{O {VI}}} opacity excess in the low η sample on scales of ∼10 cMpc for HE 2347-4342 at \bar{z}≈ 2.6, consistent with a large-scale excess in hard UV photons. However, for HS 1700 + 6416 at \bar{z}≈ 2.5 we find that the measured {{O {VI}}} absorption is not sensitive to differences in η. HS 1700 + 6416 also shows a relative absence of {{O {VI}}} overall, which is 6σ inconsistent with that of HE 2347-4342. This implies UV background inhomogeneities on ≳200 cMpc scales, hard UV regions having internal ionization structure on ∼10 cMpc scales, and soft UV regions showing no such structure. Furthermore, we perform the pixel optical depth search for oxygen on the {{He {II}}} Gunn-Peterson trough of HE 2347-4342 and find results consistent with post-{{He {II}}}-reionization conditions.