  The physical basis for the belief that the abundant old dwarf galaxies seen
in present-day galaxy clusters formed before reionization of the universe is
very compelling, because (1) the observed faint end slope of the galaxy
luminosity function ascends from a shallower slope at brighter magnitudes and
tracks that of dark matter halo mass function, and (2) that steep power-law
slope is observed to extend all the way to galaxies of inferred velocity
dispersion of ~10km/s. We then show that the number of ionizing photons emitted
by these dwarf galaxies should be able to produce an H II region at least as
large as each proto-cluster region, occupying at least 20% of the entire IGM
volume at z >= 6. A more likely scenario is that our estimate of the ionizing
photon production rate based on these dwarfs under-estimates the true rate from
proto-clusters by a factor of ~5 for a variety of physically plausible reasons,
in which case a fully self- consistent picture emerges. Specific implications
include (1) Strong clustering of sources would collectively produce very large
individual H II regions of sizes up to ~100Mpc before overlapping with others,
having important implications for upcoming radio and CMB observations. There
should be large spatial fluctuations of neutral fraction in the intergalactic
medium up to cluster scales of ~100Mpc. We give an H II region size function.
(2) The ionizing sources at z~6 are expected to be highly biased with a bias
factor >=15. (3) Large H II regions would enable largely unattenuated
transmission of flux of a significant number of sources prior to reionization.
When more sensitive observations become available, one should expect to see
clusters of fainter sources in the vicinity of known galaxies at z>6.
