We use state-of-the-art numerical simulations to explore the observability and the expected physical properties of the progenitors of the local group galaxies at z \simeq 6 - 8 , within 1 billion years of the big bang .
We find that the most massive progenitors of the Milky Way ( MW ) and Andromeda ( M31 ) at z \simeq 6 and 7 are predicted to have absolute UV continuum magnitudes M _ { UV } \simeq - 17 to -18 , suggesting that their analogues lie close to the detection limits of the deepest near-infrared surveys conducted to date ( i.e .
HST WFC3/IR UDF12 ) .
This in turn confirms that the majority of currently known z \simeq 6 - 8 galaxies are expected to be the seeds of present-day galaxies which are more massive than L _ { * } spirals .
We also discuss the properties of the local-group progenitors at these early epochs , extending down to absolute magnitudes M _ { UV } \simeq - 13 .
The most massive MW/M31 progenitors at z \simeq 7 have stellar masses M _ { * } \simeq 10 ^ { 7.5 - 8 } M _ { \odot } , stellar metallicities Z _ { * } \sim 3 - 6 \% Z _ { \odot } , and predicted observed UV continuum slopes \beta \simeq - 2.4 to -2.5 .