Comparing the Quenching Times of Faint M31 and Milky Way Satellite Galaxies

Daniel R. Weisz, Nicolas F. Martin, Andrew E. Dolphin, Saundra M. Albers, Michelle L.M. Collins, Annette M.N. Ferguson, Geraint F. Lewis, Dougal Mackey, Alan McConnachie, R. Michael Rich, Evan D. Skillman

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30 Scopus citations


We present the star formation histories (SFHs) of 20 faint M31 satellites (-12 ≲ M V ≲-6) that were measured by modeling sub-horizontal branch depth color-magnitude diagrams constructed from Hubble Space Telescope (HST) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration 3-6 Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent (<3 Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SFHs: one with exponentially declining SFHs (τ ∼ 2 Gyr) and one with rising SFHs with abrupt quenching. We speculate how these two groups could be related to scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury survey of M31 satellites will provide well-constrained ancient SFHs to go along with the quenching times we measure here. The discovery and characterization of M31 satellites with M V ⪆-6 would help quantify the relative contributions of reionization and environment to quenching of the lowest-mass satellites.

Original languageEnglish (US)
Article numberL8
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Nov 1 2019

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© 2019. The American Astronomical Society. All rights reserved.


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