Testing the near-far connection with FIRE simulations: Inferring the stellar mass function of the proto-Local Group at z > 6 using the fossil record of present-day galaxies

The shape of the low-mass (faint) end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at z 6 is an open question for understanding which galaxies primarily drove cosmic reionization. Resolved photometry of Local Group low-mass galaxies allows us to reconstruct their star formation histories, stellar masses, and UV luminosities at early times, and this fossil record provides a powerful ‘near-far’ technique for studying the reionization-era SMF/UVLF, probing orders of magnitude lower in mass than direct HST/JWST observations. Using 882 low-mass (Mₛₜₐᵣ 10⁹\, M_) galaxies across 11 Milky Way (MW)- and Local Group-analogue environments from the FIRE-2 cosmological baryonic zoom-in simulations, we characterize their progenitors at $z=6\!-\!9$, the mergers/disruption of those progenitors over time, and how well their present-day fossil record traces the high-redshift SMF. A present-day galaxy with Mₛₜₐᵣ~ 10⁵\, M_ (~ 10⁹\, M_) had ≈ 1 (≈ 30) progenitors at z≈ 7, and its main progenitor comprised ≈ 100~\ per\ cent (≈ 10~\ per\ cent) of the total stellar mass of all its progenitors at z≈ 7. We show that although only ~ 15~\ per\ cent of the early population of low-mass galaxies survives to present day, the fossil record of surviving Local Group galaxies accurately traces the low-mass slope of the SMF at z ~ 6 \!-\! 9. We find no obvious mass dependence to the mergers and accretion, and show that applying this reconstruction technique to just low-mass galaxies at $z = 0$ and not the MW/M31 hosts correctly recovers the slope of the SMF down to Mₛₜₐᵣ ~ 104.5\, M at z 6. Thus, we validate the ‘near-far’ approach as an unbiased tool for probing low-mass reionization-era galaxies.