Le 25 novembre 2016
De 10h30 à 12h00
Jonathan CHARDIN
Cambridge University, UK
What are the main sources that reionized the inter-galactic medium during the first billion year of the Universe: the star forming galaxies or the active galactic nuclei (AGNs)? It has been believed for a long time that star-forming galaxies would be the main driver of reionization from the observed space density of such object compared to the much rarer AGNs at those redshifts. However, it has been shown recently that Lyman-alpha forest data probing the post-reionization Universe shows surprisingly large opacity fluctuations over rather large (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 AGNs 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 AGNs to the ionizing UV background. This improved models confirm our earlier findings that a significant (50%) contribution of ionizing photons from AGNs can explain the large reported opacity fluctuations on large scales. The inferred AGN luminosity function is thereby consistent with recent estimates of the space density of AGNs at those redshifts. 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 AGN surveys as well as targeted searches for the predicted strong transverse AGN proximity effect whould illuminate the origin of the observed large scale opacity fluctuations. They would allow to substantiate whether UV fluctuations due to AGNs 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.