# 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

**Authors:** Pratik J. Gandhi, Andrew Wetzel, Michael Boylan-Kolchin, Robyn E. Sanderson, Alessandro Savino, Daniel R. Weisz, Erik J. Tollerud, Guochao Sun, Claude-Andre Faucher-Giguere  
**arXiv:** [2309.09940](https://arxiv.org/abs/2309.09940)  
**Date:** 2023-09-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2309.09940v1/

## Key points

- Shape of the low-mass end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at redshifts greater than 6 is a topic of ongoing research
- Resolved photometry of low-mass galaxies in the Local Group provides valuable information about their star formation histories, stellar masses, and UV luminosities during early times
- Fossil record of low-mass galaxies offers a powerful technique known as the "near-far" approach for studying the SMF/UVLF during the reionization era
- Study utilizes cosmological baryonic zoom-in simulations to investigate 882 low-mass galaxies across Milky Way- and Local Group-analogue environments
- Results show that present-day galaxy with stellar mass around 10^5 solar masses had one progenitor at redshift 7, while a galaxy with stellar mass around 10^9 solar masses had about thirty progenitors at that same redshift
- Only about 15% of initial population of low-mass galaxies survives until present day, but their fossil record accurately traces the low-mass slope of the SMF at redshifts between 6 and 9
- No significant mass dependence observed in terms of mergers and accretion
- Reconstruction technique using resolved photometry from low-mass galaxies validates "near-far" approach for studying early galaxy formation

## Summary

The shape of the low-mass end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at redshifts greater than 6 is a topic of ongoing research in order to understand which galaxies played a significant role in cosmic reionization. Resolved photometry of low-mass galaxies in the Local Group provides valuable information about their star formation histories, stellar masses, and UV luminosities during early times. This fossil record offers a powerful technique known as the "near-far" approach for studying the SMF/UVLF during the reionization era, allowing researchers to probe galaxies with masses several orders of magnitude lower than what can be directly observed by instruments like HST/JWST. In this study, the authors utilize cosmological baryonic zoom-in simulations from the FIRE-2 project to investigate 882 low-mass galaxies (with stellar masses less than 10^9 solar masses) across 11 Milky Way- and Local Group-analogue environments. They aim to characterize these galaxies' progenitors at redshifts around 6 to 9, track their mergers and disruptions over time, and assess how well their present-day fossil record reflects the high-redshift SMF. The results show that a present-day galaxy with a stellar mass of approximately 10^5 solar masses had only one progenitor at around redshift 7, while a galaxy with a stellar mass of around 10^9 solar masses had about thirty progenitors at that same redshift. Furthermore, the main progenitor comprised nearly all of the total stellar mass for both types of galaxies at redshift 7. Although only about 15% of the initial population of low-mass galaxies survives until present day, their fossil record accurately traces the low-mass slope of the SMF at redshifts between 6 and 9. Interestingly, no significant mass dependence is observed in terms of mergers and accretion. Additionally, the authors demonstrate that applying this reconstruction technique solely to low-mass galaxies at redshift 0 (present day) without considering the Milky Way/M31 hosts correctly recovers the slope of the SMF down to a stellar mass of approximately 10^4.5 solar masses at redshifts greater than 6. This validates the "near-far" approach as an unbiased tool for studying low-mass galaxies during reionization era. In conclusion, this study expands our understanding of the low-mass end of galaxy SMF/UVLF at high redshifts and provides insights into which galaxies may have played a significant role in cosmic reionization. The findings support using resolved photometry from Local Group low-mass galaxies as an effective tool for studying early galaxy formation and evolution.