  We present the analysis of deep HST multi-band imaging of the BDF field
specifically designed to identify faint companions around two of the few
Ly-alpha emitting galaxies spectroscopically confirmed at z~7 (Vanzella et al.
2011). Although separated by only 4.4 proper Mpc these galaxies cannot generate
HII regions large enough to explain visibility of their Ly-alpha line, thus
requiring a population of fainter ionizing sources in their vicinity. We use
deep HST and VLT-Hawk-I data to select z~7 Lyman break galaxies around the
emitters. We select 6 new robust z~7 LBGs at Y~26.5-27.5 whose average spectral
energy distribution is consistent with the objects being at the redshift of the
close-by Ly-alpha emitters. The resulting number density of z~7 LBGs in the BDF
field is a factor ~3-4 higher than expected in random pointings of the same
size. We compare these findings with cosmological hydrodynamic plus radiative
transfer simulations of a universe with a half neutral IGM: we find that indeed
Ly-alpha emitter pairs are only found in completely ionized regions
characterized by significant LBG overdensities. Our findings match the
theoretical prediction that the first ionization fronts are generated within
significant galaxy overdensities and support a scenario where faint, "normal"
star-forming galaxies are responsible for reionization.
