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 similar or equal to 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 similar or equal to 6 and 7 are predicted to have absolute ultraviolet (UV) continuum magnitudes M-UV similar or equal to -17 to -18, suggesting that their analogues lie close to the detection limits of the deepest near-infrared (IR) surveys conducted to date [i.e. Hubble Space Telescope Wide Field Camera 3/IR Ultra Deep Field (UDF) 12]. This in turn confirms that the majority of currently known z similar or equal to 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 similar or equal to -13. The most massive MW/M31 progenitors at z similar or equal to 7 have stellar masses M-* similar or equal to 10(7.5-8) M-circle dot, stellar metallicities Z(*) similar to 3-6 per cent Z(circle dot), and predicted observed UV continuum slopes beta similar or equal to -2.4 to -2.5.
- galaxies: evolution
- galaxies: high-redshift
- Local Group
- galaxies: luminosity function, mass function
- cosmology: theory