# Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era

## TL;DR

This paper links ultra-faint dwarf galaxies in the Local Group to their ancient star-forming ancestors during reionization, revealing their significant role in early galaxy formation and reionization, with implications for observational strategies.

## Contribution

It demonstrates that the faintest UFDs originated from ultra-violet faint galaxies during reionization, impacting our understanding of galaxy formation and reionization history.

## Key findings

- UFD ancestors had ${
m M}_{
m UV} \,\sim -3$ to $-6$ during reionization.
- Faint galaxies contributed over 50% of UV flux during reionization.
- High-redshift UVLFs imply increased star formation and lower escape fractions.

## Abstract

Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; ${\rm M}_{\rm V}$ $\sim -2$ or ${\rm M}_{\star}$ $\sim 10^{2}$ at $z=0$) had ultra-violet (UV) luminosities of ${\rm M}_{\rm UV}$ $\sim -3$ to $-6$ during reionization ($z\sim6-10$). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep ($\alpha\lesssim-2$) to ${\rm M}_{\rm UV}$ $\sim -3$, then: (i) the ancestors of UFDs produced $>50$% of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are $>$2 times lower than currently-adopted values; (iii) direct HST and JWST observations may detect only $\sim10-50$% of the UV light from galaxies; (iv) the cosmic star formation history increases by $\gtrsim4-6$ at $z\gtrsim6$. Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to ${\rm M}_{\rm UV}$ $\sim -3$ during reionization.