  We present a novel method to investigate cosmic reionization, using joint
spectral information on high redshift Lyman Alpha Emitters (LAE) and quasars
(QSOs). Although LAEs have been proposed as reionization probes, their use is
hampered by the fact their Ly{\alpha} line is damped not only by intergalactic
HI but also internally by dust. Our method allows to overcome such degeneracy.
First, we carefully calibrate a reionization simulation with QSO absorption
line experiments. Then we identify LAEs in two simulation boxes at z=5.7 and
z=6.6 and we build synthetic images/spectra of a prototypical LAE. At redshift
5.7, we find that the Ly{\alpha} transmissivity (T_LAE) ~ 0.25, almost
independent of the halo mass. This constancy arises from the conspiracy of two
effects: (i) the intrinsic Ly{\alpha} line width and (ii) the infall peculiar
velocity. At higher redshift, z=6.6, where the transmissivity is instead
largely set by the local HI abundance and LAE transmissivity consequently
increases with halo mass from 0.15 to 0.3. Although outflows are present, they
are efficiently pressure-confined by infall in a small region around the LAE;
hence they only marginally affect transmissivity. Finally, we cast LOS
originating from background QSOs passing through foreground LAEs at different
impact parameters, and compute the quasar transmissivity (T_QSO). At smaller
impact parameters, d < 1 cMpc, a positive correlation between T_QSO and halo
mass is found at z = 5.7, which tends to become less pronounced (i.e. flatter)
at larger distances. By cross-correlating T_LAE and T_QSO, we can obtain a HI
density estimate unaffected by dust. At z= 5.7, the cross-correlation is
relatively weak,whereas at z = 6.6 we find a clear positive correlation. We
conclude by briefly discussing the perspectives for the application of the
method to existing and forthcoming data.
