# Star Formation Histories of Ultra-Faint Dwarf Galaxies: environmental differences between Magellanic and non-Magellanic satellites?

**Authors:** Elena Sacchi, Hannah Richstein, Nitya Kallivayalil, Roeland van der Marel, Mattia Libralato, Paul Zivick, Gurtina Besla, Thomas M. Brown, Yumi Choi, Alis Deason, Tobias Fritz, Marla Geha, Puragra Guhathakurta, Myoungwon Jeon, Evan Kirby, Steven R. Majewski, Ekta Patel, Joshua D. Simon, Sangmo Tony Sohn, Erik Tollerud  
**arXiv:** [2108.04271](https://arxiv.org/abs/2108.04271)  
**Date:** 2021-08-09  
**Summary page:** https://www.summarizepaper.com/en/paper/2108.04271v1/

## Key points

- The study presents color-magnitude diagrams and star formation histories of seven ultra-faint dwarf galaxies.
- Each galaxy created at least 80% of its stellar mass by $z\sim6$.
- All galaxies show quenching times older than 11.5 Gyr ago, consistent with reionization suppressing the star formation of small dark matter halos.
- Differences in the SFHs of candidate Magellanic Cloud satellites compared to other Milky Way satellites suggest that Magellanic satellites show quenching times about 600 Myr more recent on average than those of other Milky Way satellites.
- This research provides insight into how environment and reionization can impact low-mass galaxies such as these ultra-faint dwarf galaxies.
- The observations presented here will be an important baseline for follow-up imaging with HST and the James Webb Space Telescope to measure both bulk PMs and internal motions as well as spectroscopic surveys to improve metallicity constraints for more precise age derivations on these very interesting galaxies.
- The study was associated with the HST Treasury Program 14734 (PI Kallivayalil), supported by NASA through grants from the Space Telescope Science Institute.
- AW received support from NASA ATP grants 80NSSC18K1097 and 80NSSC20K0513, a Scialog Award from the Heising-Simons Foundation, and a Hellman Fellowship.
- ENK gratefully acknowledges support from a Cottrell Scholar award administered by the Research Corporation for Science Advancement.

## Summary

This study presents the color-magnitude diagrams and star formation histories (SFHs) of seven ultra-faint dwarf galaxies: Horologium 1, Hydra 2, Phoenix 2, Reticulum 2, Sagittarius 2, Triangulum 2, and Tucana 2. The SFHs were derived from high-precision Hubble Space Telescope photometry. The findings indicate that each galaxy created at least 80% of its stellar mass by $z\sim6$. Furthermore, all galaxies show quenching times older than 11.5 Gyr ago which is consistent with the scenario in which reionization suppresses the star formation of small dark matter halos. However, this analysis also reveals some differences in the SFHs of candidate Magellanic Cloud satellites compared to other Milky Way satellites. These differences suggest that Magellanic satellites show quenching times about 600 Myr more recent on average than those of other Milky Way satellites. This finding is consistent with theoretical models suggesting that satellite SFHs may depend on their host environment at early times. This research provides insight into how environment and reionization can impact low-mass galaxies such as these ultra-faint dwarf galaxies. A detailed comparison between UFDs residing in different environments at early times could reveal variations in the ionization field over large scales. The observations presented here will be an important baseline for follow-up imaging with HST and the James Webb Space Telescope to measure both bulk PMs and internal motions as well as spectroscopic surveys to improve metallicity constraints for more precise age derivations on these very interesting galaxies. This study was associated with the HST Treasury Program 14734 (PI Kallivayalil), supported by NASA through grants from the Space Telescope Science Institute. AW received support from NASA ATP grants 80NSSC18K1097 and 80NSSC20K0513, a Scialog Award from the Heising-Simons Foundation, and a Hellman Fellowship. This material is based upon work supported by the National Science Foundation under grant No. AST-1847909 ENK gratefully acknowledges support from a Cottrell Scholar award administered by the Research Corporation for Science Advancement.