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The Hubble Space Telescope Survey of M31 Satellite Galaxies. II. The Star Formation Histories of Ultrafaint Dwarf Galaxies

Savino, Alessandro; Weisz, Daniel R.; Skillman, Evan D.; Dolphin, Andrew; Cole, Andrew A.; Kallivayalil, Nitya; Wetzel, Andrew; Anderson, Jay; Besla, Gurtina; Boylan-Kolchin, Michael; Brown, Thomas M.; Bullock, James S.; Collins, Michelle L. M.; Cooper, M. C.; Deason, Alis J.; Dotter, Aaron L.; Fardal, Mark; Ferguson, Annette M. N.; Fritz, Tobias K.; Geha, Marla C.; Gilbert, Karoline M.; Guhathakurta, Puragra; Ibata, Rodrigo; Irwin, Michael J.; Jeon, Myoungwon; Kirby, Evan N.; Lewis, Geraint F.; Mackey, Dougal; Majewski, Steven R.; Martin, Nicolas; McConnachie, Alan; Patel, Ekta; Rich, R. Michael; Simon, Joshua D.; Sohn, Sangmo Tony; Tollerud, Erik J.; van der Marel, Roeland P.

The Hubble Space Telescope Survey of M31 Satellite Galaxies. II. The Star Formation Histories of Ultrafaint Dwarf Galaxies Thumbnail


Alessandro Savino

Daniel R. Weisz

Evan D. Skillman

Andrew Dolphin

Andrew A. Cole

Nitya Kallivayalil

Andrew Wetzel

Jay Anderson

Gurtina Besla

Michael Boylan-Kolchin

Thomas M. Brown

James S. Bullock

Michelle L. M. Collins

M. C. Cooper

Aaron L. Dotter

Mark Fardal

Annette M. N. Ferguson

Tobias K. Fritz

Marla C. Geha

Karoline M. Gilbert

Puragra Guhathakurta

Rodrigo Ibata

Michael J. Irwin

Myoungwon Jeon

Evan N. Kirby

Geraint F. Lewis

Dougal Mackey

Steven R. Majewski

Nicolas Martin

Alan McConnachie

Ekta Patel

R. Michael Rich

Joshua D. Simon

Sangmo Tony Sohn

Erik J. Tollerud

Roeland P. van der Marel


We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD; M V > − 7.0, 4.9<log10(M*(z=0)/M⊙)<5.5 ) 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 by z = 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, And xiii, which formed only 10% of its stellar mass by z = 5, and 75% in a rapid burst at z ∼ 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 × 104 M ⊙) are likely quenched by reionization, whereas more-massive M31 UFDs (M *(z = 5) ≳ 105 M ⊙) 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.


Savino, A., Weisz, D. R., Skillman, E. D., Dolphin, A., Cole, A. A., Kallivayalil, N., …van der Marel, R. P. (2023). The Hubble Space Telescope Survey of M31 Satellite Galaxies. II. The Star Formation Histories of Ultrafaint Dwarf Galaxies. Astrophysical Journal, 956(2), Article 86.

Journal Article Type Article
Acceptance Date Aug 25, 2023
Online Publication Date Oct 11, 2023
Publication Date Oct 1, 2023
Deposit Date Oct 18, 2023
Publicly Available Date Oct 18, 2023
Journal The Astrophysical Journal
Print ISSN 0004-637X
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 956
Issue 2
Article Number 86
Keywords Dwarf galaxies, Hertzsprung Russell diagram, Reionization, Galaxy quenching, Andromeda Galaxy
Public URL


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