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Lyman-alpha forest data probing the post-reionization Universe showssurprisingly large opacity fluctuations over rather large ($\ge$50 comovingMpc/h) spatial scales. We model these fluctuations using a hybrid approachutilizing the large volume Millennium simulation to predict the spatialdistribution of QSOs combined with smaller scale full hydrodynamical simulationperformed with RAMSES and post-processed with the radiative transfer code ATON.We produce realictic mock absorption spectra that account for the contributionof galaxies and QSOs to the ionising UV background. This improved modelsconfirm our earlier findings that a significant ($\ge$50%) contribution ofionising photons from QSOs can explain the large reported opacity fluctuationson large scales. The inferred QSO luminosity function is thereby consistentwith recent estimates of the space density of QSOs at this redshift. Oursimulations still somewhat struggle, however, to reproduce the very long (110comoving Mpc/h) high opacity absorption through observed in ULAS J0148+0600,perhaps suggesting an even later end of reionization than assumed in ourpreviously favoured model. Medium-deep/medium area QSO surveys as well astargeted searches for the predicted strong transverse QSO proximity effectwhould illuminate the origin of the observed large scale opacity fluctuations.They would allow to substantiate whether UV fluctuations due to QSO are indeedprimarily responsible, or whether significant contributions from other recentlyproposed mechansims such as large scale fluctuations in temperature and meanfree path (even in the absence of rare bright sources) are required.

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