The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

Abstract We present deep Hubble Space Telescope (HST) imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1’s evolutionary drivers, we resolve stars in F8D1’s central ∼1 kpc and in a parallel field ∼6 kpc along its major axis to deep photometric limits, reaching below the red clump. We also image eight shallower fields along F8D1’s major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst ∼2 Gyr ago and a smaller burst ∼500 Myr ago, which likely formed F8D1’s nuclear star cluster. In the shallow fields, using the ratio of evolved asymptotic giant branch and red giant branch stars out to ∼13 kpc along F8D1’s known stellar stream, we confirm that F8D1 was globally star-forming until at least ∼2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of ∼1.3 × 10 8 M ⊙ , with an average [M/H] ∼ −0.8. We compare F8D1’s properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.

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Publication Details

Journal
The Astrophysical Journal
Published
2026-08-18
DOI
https://doi.org/10.3847/1538-4357/ae8192
Primary Topic
Astronomy and Astrophysical Research
Type
article
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article

The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

Benjamin F. Williams, Antonela Monachesi, Kristen B. W. McQuinn, Andrew E. Dolphin et al.
The Astrophysical Journal
Astronomy and Astrophysical Research
article

The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

Benjamin F. Williams, Antonela Monachesi, Kristen B. W. McQuinn, Andrew E. Dolphin, Colin T. Slater, Adam Smercina, Daniel R. Weisz, Andrew R. Wetzel, Eric F. Bell, Puragra Guhathakurta, Elisa Toloba, Richard D’Souza, Jeremy Bailin, Roelof S. de Jong, T K Chan, Sarah Pearson, Julianne J. Dalcanton, Benjamin N. Velguth
article en

Abstract

Abstract We present deep Hubble Space Telescope (HST) imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1’s evolutionary drivers, we resolve stars in F8D1’s central ∼1 kpc and in a parallel field ∼6 kpc along its major axis to deep photometric limits, reaching below the red clump. We also image eight shallower fields along F8D1’s major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst ∼2 Gyr ago and a smaller burst ∼500 Myr ago, which likely formed F8D1’s nuclear star cluster. In the shallow fields, using the ratio of evolved asymptotic giant branch and red giant branch stars out to ∼13 kpc along F8D1’s known stellar stream, we confirm that F8D1 was globally star-forming until at least ∼2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of ∼1.3 × 10 8 M ⊙ , with an average [M/H] ∼ −0.8. We compare F8D1’s properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.

The Astrophysical JournalVol. 1007(2)
Rutgers, The State University of New Jersey (US), Dartmouth College (US), Tufts University (US), University of Copenhagen (DK), Space Telescope Science Institute (US), University of Arizona (US), University of Alabama (US), Chinese University of Hong Kong (HK), University of California, Santa Cruz (US), University of Washington (US), University of Michigan (US), Leibniz Institute for Astrophysics Potsdam (DE), Vatican Secret Archives (VA), University of California System (US), Raytheon Technologies (Netherlands) (NL), Flatiron Health (United States) (US), Dartmouth Hospital (GB), Flatiron Institute (US), University of La Serena (CL)
Openalex Percentile: Top 98%
Astronomy and Astrophysical Research
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