Large scale opacity fluctuations in the Lyman alpha forest: evidence for QSOs dominating the ionising UV background at z ~ 5.5-6 ?

Abstract

Lyman-alpha forest data probing the post-reionization Universe shows surprisingly large opacity fluctuations over rather large ($\ge$50 comoving Mpc/h) spatial scales. We model these fluctuations using a hybrid approach utilizing the large volume Millennium simulation to predict the spatial distribution of QSOs combined with smaller scale full hydrodynamical simulation performed with RAMSES and post-processed with the radiative transfer code ATON. We produce realictic mock absorption spectra that account for the contribution of galaxies and QSOs to the ionising UV background. This improved models confirm our earlier findings that a significant ($\ge$50%) contribution of ionising 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 (110 comoving Mpc/h) 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 whould 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 mechansims such as large scale fluctuations in temperature and mean free path (even in the absence of rare bright sources) are required.