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Abstract We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD;MV> − 7.0, ) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass byz= 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, Andxiii, which formed only 10% of its stellar mass byz= 5, and 75% in a rapid burst atz∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This “young” UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs (M*(z= 5) ≲ 5 × 104M⊙) are likely quenched by reionization, whereas more-massive M31 UFDs (M*(z= 5) ≳ 105M⊙) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs.
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This content will become publicly available on August 13, 2026
Survivors and Zombies: The Quenching and Disruption of Satellites Around Milky Way Analogs
Abstract It is necessary to understand the full accretion history of the Milky Way in order to contextualize the properties of observed Milky Way satellite galaxies and the stellar halo. This paper compares the dynamical properties and star formation histories of surviving and disrupted satellites around Milky Way–like galaxies using theD.C. Justice Leaguesuite of very high-resolution cosmological zoom-in simulations of Milky Way analogs and their halo environments. We analyze the full census of galaxies accreted within the past 12 Gyr, including both surviving satellites atz= 0, and dwarf galaxies that disrupted and merged with the host prior toz= 0. Our simulations successfully reproduce the trends inM*−[Fe/H]−[α/Fe] observed in surviving Milky Way satellites and disrupted stellar streams, indicating earlier star formation for disrupted progenitors. We find the likelihood and timescales for quenching and disruption are strongly correlated with the mass and time of infall. In particular, none of the galaxies accreted more than 12 Gyr ago survived, and only 20% of all accreted galaxies withM* > 108M⊙survive. Additionally, satellites with highly radial trajectories are more likely to quench and disrupt. Disruption proceeds quickly for ≥106M⊙satellites accreted 10–12 Gyr ago, often on timescales similar to the ∼300 Myr snapshot spacing. For high-mass satellites, the disruption timescale is faster than the quenching timescale. As a result, 92% of disrupted galaxies remain star forming up until disruption. In contrast, ultrafaint dwarfs (UFDs) tend to quench prior to accretion, and 94% of UFDs accreted up to 12 Gyr ago survive atz= 0.
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- Award ID(s):
- 1848107
- PAR ID:
- 10660290
- Publisher / Repository:
- Institute of Physics
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 989
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 178
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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