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 dominant 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 atz ≈ 2.6, consistent with a large-scale excess in hard UV photons. However, for HS 1700+6416 atz ≈ 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.

On the AGN nature of two UV bright sources at z_spec~5.5 in the CANDELS fields: an update of the AGN space density at M1450~-22.5

“…These measurements are in agreement with the predictions based on the stochasticity of the space density of ionizing sources such as high-z AGNs and the partial reionization of optically thick absorbers near the completion of the reionization process (Compostella et al 2014;Chardin et al 2015Chardin et al , 2017Madau 2017). In practice, strong fluctuations in HeII absorption could be due to spatial variation of the hard UV background in an already ionized medium at z ∼ 3 (Morrison et al 2019). A soft UV background produced by a homogeneous distribution of ubiquitous ultra-faint SFGs at z > 10 is in tension with such measurements.…”

Probing Large-scale UV Background Inhomogeneity Associated with Quasars using Metal Absorption

“…(vii) The results support the findings of Morrison et al (2019) that O shows inhomogeneity on scales of several 10s of comoving megaparsecs, reaching even 100 cMpc h −1 . This work goes a step further, however, confirming expected connection to the quasar population.…”

“…On the other hand three of our > 3 detections are not marginal and our O results are particularly robust. The overall the picture presented in Morrison et al (2019) of inhomogeneities in the O on several 10s of cMpc is strongly supported here, despite the very different data and analysis methods used. Indeed, we are able to go further than that work and confirm that these inhomogeneities are at least correlated with quasar positions.…”

“…The phase change due to He reionization can be probed with absorption in the He Lyman-(303.78Å; hereafter He Ly-) forest, however, very few He Ly-forests are observable. Recent work (Morrison et al 2019) showed that the large-scale distribution of high ionization metals such as O probed through absorption can compliment and (where necessary) be used as proxy for He Ly-. These measurements showed inhomogeneity on scales from ∼ 10 cMpc and 200 cMpc in the UV background irrespective of the observability of the sources of ionization.…”

Evolution of CIV Absorbers I. The Cosmic Incidence

“…Theoretical models have often assumed the UVB is spatially homogeneous and externally imposed (e.g., Oppenheimer et al 2010;van de Voort et al 2011;Keating et al 2016;Rahmati et al 2016), but such models have often been unsuccessful in reproducing observed properties of C iv absorbers at z ≥ 4 (Oppenheimer et al 2009;Rahmati et al 2016;Finlator et al 2016;Finlator et al 2018). High redshift observational studies have also identified a need for variations in the local ionizing radiation field around absorbers (e.g., D 'Odorico et al 2013;Boksenberg & Sargent 2015;Morrison et al 2019).…”