Hubble observations shed more light on the star formation history of a nearby ultra-diffuse galaxy
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Using the Hubble Space Telescope (HST), astronomers have observed a nearby ultra-diffuse galaxy designated F8D1. The , presented July 17 on the pre-print server arXiv, deliver important insights into the star formation history of this galaxy.
In general, ultra-diffuse galaxies (UDGs) are extremely-low-density galaxies. Observations show that the largest UDGs are similar in size to the Milky Way, but have only about 1% as many stars as our galaxy. It still remains a mystery why these faint but large galaxies are not ripped apart by the tidal field of their host clusters, hence, astronomers are interested in studying UDGs in detail in order to resolve this puzzle.
That is why a group of astronomers led by Adam Smercina of the Space Telescope Science Institute in Baltimore, Maryland, have employed HST's Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS) to investigate F8D1—an UDG discovered in 1998, which is a satellite of the M81 group of galaxies. F8D1 showcases unique properties among other galaxy populations and is perceived as a prototypical UDG.
"We have used new HST observations to infer the star formation history of the nearby UDG, F8D1, over the past six billion years, using detections of individual stars to below the Red Clump in a central WFC3 field and an outer ACS field," the researchers wrote in the paper.
Using HST, Smercina's team were able to measure the properties of F8D1's main body and stellar stream. Their observations represent one of the most detailed case studies of a UDG conducted so far.
First of all, the observations allowed the astronomers to identify three distinct periods of star formation in F8D1. The first one started over six billion years ago, when about 80% of the galaxy's stars formed.
This period was followed by a burst of star formation about 2–2.5 billion years ago, which formed approximately 11% of the stars in the central field and about 39% in the outer field. Afterward, a smaller burst occurred some 500 million years ago, during which the F8D1's nuclear star cluster was formed and also a small percentage of the galaxy's stars were born.
Furthermore, the observations indicate that F8D1, including its core and tidal stream (out to 42,400 light years), experienced a global shutdown of star formation less than two billion years ago. This suggests that F8D1 was globally star-forming two billion years ago.
According to the paper, the progenitor mass of F8D1 was estimated to be 133 million solar masses. The astronomers noted that 25–40% of the stellar mass of this UDG is currently unbound in this extensive tidal stream.
Summing up the results, the authors of the paper underline that the presence of significant star formation late in the history of F8D1, together with typical globular cluster population, provide some hints regarding the origin of this UDG.
The findings exclude the possibility that F8D1 may be a "failed" galaxy, as some studies suggest that UDGs are stellar groupings that experienced violent early star-forming episodes, which completely expelled their gas reservoirs before they could form a dense baryonic component.
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More information: Adam Smercina et al, The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite, F8D1, arXiv (2025).
Journal information: arXiv
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