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 ; { M } _ { V } \sim - 2 or { M } _ { \star } \sim 10 ^ { 2 } at z = 0 ) had ultra-violet ( UV ) luminosities of { M } _ { UV } \sim - 3 to -6 during reionization ( z \sim 6 - 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 { M } _ { 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 \sim 10 - 50 % of the UV light from galaxies ; ( iv ) the cosmic star formation history increases by \gtrsim 4 - 6 at z \gtrsim 6 .
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 { M } _ { UV } \sim - 3 during reionization .