Large-scale opacity fluctuations in the Lyα forest: evidence for QSOs dominating the ionizing UV background atz∼ 5.5–6?

Abstract

Lyman-alpha forest data probing the post-reionization Universe shows surprisingly large opacity fluctuations over rather large ( 50h −1 comoving Mpc) spatial scales. We model these fluctuations using a hybrid approach utilizing the large volume Millennium simulation to predict the spatial distribution of QSOs combined with smaller scale full hydrodynamical simulation performed with RAMSES and post-processed with the radiative transfer code ATON. We produce realistic mock absorption spectra that account for the contribution of galaxies and QSOs to the ionizing UV background. These improved models confirm our earlier findings that a significant ( 50%) contribution of ionizing photons from QSOs can explain the large reported opacity fluctuations on large scales. The inferred QSO luminosity function is thereby consistent with recent estimates of the space density of QSOs at this redshift. Our simulations still somewhat struggle, however, to reproduce the very long (110h −1 comoving Mpc) high opacity absorption through observed in ULAS J0148+0600, perhaps suggesting an even later end of reionization than assumed in our previously favoured model. Medium-deep/medium area QSO surveys as well as targeted searches for the predicted strong transverse QSO proximity effect would illuminate the origin of the observed large scale opacity fluctuations. They would allow to substantiate whether UV fluctuations due to QSO are indeed primarily responsible, or whether significant contributions from other recently proposed mechanisms such as large scale fluctuations in temperature and mean free path (even in the absence of rare bright sources) are required.

Constraining the contribution of active galactic nuclei to reionization

“…In addition, they might also support the early and extended Helium reionisation observed by Worseck et al (2016). Independently, Chardin et al (2017) argued that the large scale opacity fluctuations in the Lyα forest measured by Becker et al (2015) could be explained if AGN dominate the ionising UV background at z ∼ 6 (see also Chardin et al 2015).…”

“…We include this in order to check whether including all faint AGN would allow reionisation completion to be consistent with neutral fraction and optical depth constraints. This means that we are effectively studying an optimal case for reionisation by AGN, since those fainter than M1450 > −18 might already be a part of the galaxy population as discussed in Chardin et al (2017), due to the overlap between the galaxy and AGN luminosity functions at this faint limit. Applying a magnitude cut-off would suppress the ionising emissivity and further delay reionisation.…”

Probing inhomogeneity in the helium ionizing UV background

“…We do not attempt to measure η on line-of-sight scales larger than 20 cMpc due to concerns that the measured η variation will be sensitive to H i continuum normalisation (analogous to how large-scale structures are not measured along lines-of-sight on these larger scales e.g. Chabanier et al 2018). Our results for the fiducial 50/50 case seem, on face value, to indicate that inhomogeneities in the UV background seen at 10 cMpc and are not present on scales of 20 cMpc.…”

Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function

“…In this picture, quasars mainly maintain the post-reionization UV background (e.g. Becker & Bolton 2013;Kulkarni et al 2019), with some additional role in determining the patchiness of the end of the process of reionization (Chardin et al 2015(Chardin et al , 2017Kakiichi et al 2018). This is for instance supported by observations of faint lensed galaxies behind clusters, indicating that the faint-end of the galaxy UV luminosity function (LF) might be steep, with no sign of any turn-over brighter than M UV −15 (e.g.…”