Discovery of the faintest Milky Way satellite.
View of the galaxy UMa3/U1 in several astronomical surveys : PanSTARRS DR1 (top left), DESI Legacy survey (top right), DECaLS DR5 (bottom left), et SDSS9 (bottom right). The red ellipse corresponds to the claimed size in the original paper.
Galaxies are generally thought of as majestic collections of billions of stars, often arranged on a swirling disk teeming with spiral patterns. But galaxies span a wide range of masses : the most massive ones contain hundreds of billions of Suns, while at the other end, galaxies as faint as a few thousands Suns are known.
An international team of scientists, including researching from the Strasbourg astronomical Observatory, now reports in The Astrophysical Journal the discovery of an ancient group of stars orbiting the Milky Way in data obtained as part of the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) at the Canada-France-Hawaii telescope (CFHT). The newly discovered satellite (which was confirmed with follow-up data from the W.M Keck Observatories) consists of only a couple of dozen bright stars spread over a volume just 10 light years across. This is minute compared to the Milky Way, which contains over ten billion stars, and measures a hundred thousand light years across.
The object, named Ursa Major 3/UNIONS 1 or UMa3/U1 for short, consists of stars that are more than 10 billion years old. It is either the faintest ancient star cluster known to date, or the faintest and closest known dwarf galaxy ever discovered.
If UMa3/U1 is a simple cluster of stars, we are most likely catching it during the very final stages of its existence, as the tidal forces produced by the Milky Way finish pulling it apart. This destruction process has been constantly happening since the formation of our Galaxy and such events have helped build the surroundings of the Milky Way, its stellar “halo”. The discovery of UMa3/U1 in its final throes will help understand how often such events still happen today and what are the limits of stellar clusters.
The latter scenario, with UMa3/U1 being an extreme dwarf galaxy, is the most exciting, since the presence of very faint, ancient, dark matter-dominated satellites is a cornerstone prediction of the currently favored cosmological model (ΛCDM). ΛCDM predicts that galaxies like the Milky Way have accreted hundreds of satellites during their formation and assembly. Confirming the presence of dark matter in UMa3/U1 is therefore critical for elucidating its origin. Direct confirmation requires stellar spectra of exquisite quality taken over time, which are not yet available.
Whatever the true nature of UMa3/U1, the very discovery of such an extreme satellite of the Milky Way is testament to the quality of the photometric mapping provided by UNIONS and that yet more satellites of the Milky Way still await discovery.
Article: The Discovery of the Faintest Known Milky Way Satellite Using UNIONS, Smith et al., 2024, ApJ, 961, 92.
Scientific contact: Nicolas Martin (CNRS), nicolas.martin@astro.unistra.fr