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

Abstract

ABSTRACT The shape of the low-mass (faint) end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at $z \gtrsim 6$ 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 ($M_{\rm star}\lesssim 10^{9}\, \rm {M_\odot }$) 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 $M_{\rm star}\sim 10^5\, \rm {M_\odot }$ ($\sim 10^9\, \rm {M_\odot }$) had $\approx 1$ ($\approx 30$) progenitors at $z\approx 7$, and its main progenitor comprised $\approx 100~{{\ \rm per\ cent}}$ ($\approx 10~{{\ \rm per\ cent}}$) of the total stellar mass of all its progenitors at $z\approx 7$. We show that although only $\sim 15~{{\ \rm per\ cent}}$ 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 $z \sim 6 \!-\! 9$. 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 $z \gtrsim 6$. Thus, we validate the ‘near-far’ approach as an unbiased tool for probing low-mass reionization-era galaxies.