2019MNRAS.489..868M

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.