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 $\eta$ (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 $\eta$ samples filtered on various scales. This filtering scale is intended to represent the dominant scale of any coherent oxygen excess/deficit. We find a $2\sigma$ detection of an O VI opacity excess in the low $\eta$ sample on scales of $\sim$10 cMpc for HE 2347-4342 at $\bar{z}\approx 2.6$, consistent with a large-scale excess in hard UV photons. However, for HS 1700+6416 at $\bar{z}\approx 2.5$ we find that the measured O VI absorption is not sensitive to differences in $\eta$. HS 1700+6416 also shows a relative absence of O VI overall, which is $6\sigma$ inconsistent with that of HE 2347-4342. This implies UV background inhomogeneities on $\gtrsim$200 cMpc scales, hard UV regions having internal ionization structure on $\sim$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.