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 reionisation. 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 reionisation-era SMF/UVLF, probing orders of magnitude lower in mass than direct HST/JWST observations. Using 882 low-mass ($M_{\rm star}\lesssim 10^{9}\, \rm {M_\odot }$) galaxies across 11 Milky Way- and Local Group-analogue environments from the FIRE-2 cosmological baryonic zoom-in simulations, we characterise 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 $M_{\rm star}\sim 10^5\, \rm {M_\odot }$ ($\sim 10^9\, \rm {M_\odot }$) had ≈1 (≈30) progenitors at z ≈ 7, and its main progenitor comprised $\approx 100~{{\%}}$ ($\approx 10~{{\%}}$) of the total stellar mass of all its progenitors at z ≈ 7. We show that although only $\sim 15~{{\%}}$ 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 $M_{\rm star} \sim 10^{4.5}\, \rm {{\rm M}_{\odot }}$ at z ≳ 6 . Thus, we validate the ‘near-far’ approach as an unbiased tool for probing low-mass reionisation-era galaxies.