Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era
  Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we
show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies
(UFDs; ${\rm M}_{\rm V}$ $\sim -2$ or ${\rm M}_{\star}$ $\sim 10^{2}$ at $z=0$)
had ultra-violet (UV) luminosities of ${\rm M}_{\rm UV}$ $\sim -3$ to $-6$
during reionization ($z\sim6-10$). The existence of such faint galaxies has
substantial implications for early epochs of galaxy formation and reionization.
If the faint-end slopes of the UV luminosity functions (UVLFs) during
reionization are steep ($\alpha\lesssim-2$) to ${\rm M}_{\rm UV}$ $\sim -3$,
then: (i) the ancestors of UFDs produced $>50$% of UV flux from galaxies; (ii)
galaxies can maintain reionization with escape fractions that are $>$2 times
lower than currently-adopted values; (iii) direct HST and JWST observations may
detect only $\sim10-50$% of the UV light from galaxies; (iv) the cosmic star
formation history increases by $\gtrsim4-6$ at $z\gtrsim6$. Significant flux
from UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced if
the high-redshift UVLF turns over. Independent of the UVLF shape, the existence
of a large population of UFDs requires a non-zero luminosity function to ${\rm
M}_{\rm UV}$ $\sim -3$ during reionization.
