Lyα leaks and reionization

Ly alpha absorption spectra of QSOs at redshifts z similar or equal to 6 show complete Gunn-Peterson absorption troughs ( dark gaps) separated by tiny leaks. The dark gaps are from the intergalactic medium (IGM) where the density of neutral hydrogen are high enough to produce almost saturated absorptions, however, where the transmitted leaks come from is still unclear so far. We demonstrate that leaking can originate from the lowest density voids in the IGM as well as the ionized apatches around ionizing sources using semi-analytical simulations. If leaks are produced in lowest density voids, the IGM must already be highly ionized, and the ionizing background should be almost uniform; in contrast, if leaks come from ionized patches, the neutral fraction of IGM should be still high, and the ionizing background is significantly inhomogeneous. Therefore, the origin of leaking is crucial to determining the epoch of inhomogeneous-to-uniform transition of the ionizing photon background. We show that the origin could be studied with the statistical features of leaks. Actually, Lya leaks can be well defined and described by the equivalent width W and the full width of half-area W-H, both of which are less contaminated by instrumental resolution and noise. It is found that the distributions of W and W-H of Ly alpha leaks are sensitive to the modelling of the ionizing background. We consider four representative models: uniform ionizing background (model 0), the photoionization rate of neutral hydrogen Gamma(HI) and the density of IGM are either linearly correlated ( model I), or anticorrelated ( model II), and Gamma(HI) is correlated with high-density peaks containing ionizing sources ( model III). Although all of these models can match to the mean of the observed effective optical depth of the IGM at z similar or equal to 6, the distributions of W and WH are very different from each other. Consequently, the leak statistics provides an effective tool to probe the evolutionary history of reionization at z similar or equal to 5-6.5. Similar statistics will also be applicable to the reionization of He II at z similar or equal to 3.