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The ages of globular clusters in our own Milky Way are known with precisionof about \pm 1 Gyr, hence their formation at redshifts z>~3 and their role inhierarchical cosmology and the reionization of the intergalactic medium remainrelatively undetermined. Here we analyze the effect of globular clusterformation on the observed rest-frame UV luminosity functions (LFs) and UVcontinuum slopes of high redshift galaxies in the Hubble Ultra Deep Fields. Wefind that the majority of present day globular clusters have formed during twodistinct epochs: at redshifts z ~ 2-3 and at redshifts z>~6. The birth ofproto-GC systems produce the steep, faint-end slopes of the galaxy LFs, andbecause the brightness of proto-GCs fades 5 Myrs after their formation, theirblue colors are in excellent agreement with observations.  Our results suggest that: i) the bulk of the old globular cluster populationwith estimated ages >~12 Gyr (about 50% of the total population), formed in therelatively massive dwarf galaxies at redshifts z>~6; ii) proto-GC formation wasan important mode of star formation in those dwarf galaxies, and likelydominated the reionization process. Another consequence of this scenario isthat some of the most massive Milky Way satellites may be faint and yetundiscovered because tidal stripping of a dominant GC population precedessignificant stripping of the dark matter halos of these satellites. Thisscenario may alleviate some remaining tensions between CDM simulations andobservations.

Two Epochs of Globular Cluster Formation from Deep Fields Luminosity Functions: Implications for Reionization and the Milky Way Satellites