Star formation histories and gas content limits of three ultra-faint dwarfs on the periphery of M31

TL;DR

This study investigates the star formation histories and gas content of three ultra-faint dwarf galaxies near M31, finding that two likely quenched early after reionization, with very low neutral gas content.

Contribution

It provides new HST and VLA observations, and introduces a Python-based SFH fitting code to analyze ultra-faint dwarfs around M31, revealing their quenching times.

Findings

Pegasus W shows extended star formation beyond reionization.

Pegasus V and Pisces VII likely quenched shortly after reionization.

All three UFDs have very low neutral hydrogen gas content.

Abstract

We present Hubble Space Telescope (HST) imaging of Pegasus V and Pisces VII, along with a re-analysis of the archival imaging of Pegasus W, and Jansky Very Large Array (VLA) neutral gas (HI) observations of all three. These three ultra-faint dwarfs (UFDs) are all within the Local Group in the approximate direction of M31. The VLA observations place stringent upper limits on their HI content, with all having $M_\mathrm{HI} < 10^4\;\mathrm{M_\odot}$. As the red giant branches of these UFDs are sparsely populated, we determined distances from the HST photometry of horizontal branch (HB) stars in comparison to a fiducial HB population (from M92), with all three falling in the range 0.7-1 Mpc. Using a new Python-based star formation history (SFH) fitting code (based on StarFISH), we derive SFHs of all three UFDs. As found previously, the best fit SFH for Pegasus W includes significant star formation well beyond the end of reionization, while the SFHs calculated for Pegasus V and Pisces VII are consistent with them having quenched shortly after reionization. These findings for the latter two objects indicate that, like those in the vicinity of the Milky Way, lower mass UFDs in the vicinity of M31 likely quenched at early times.