Using the Subaru Telescope’s wide-field camera, astronomers have discovered a previously unknown structure surrounding a tiny satellite galaxy of the Milky Way. The newly discovered structure exhibits features resembling the remnants of past galaxy mergers. This result provides compelling evidence that even extremely low-mass dwarf galaxies may have experienced mergers in their past.
Figure 1: Image of the Ursa Minor dwarf spheroidal galaxy (UMi dSph) observed with Hyper Suprime-Cam, covering three fields of view. The red dashed ellipse indicates the central region of the UMi dSph. Although the galaxy is extremely faint and difficult to identify visually, its member stars extend across the entire image. (Credit: NAOJ)
Numerous small satellite galaxies have long been gravitationally bound to the Milky Way, orbiting around it. These dwarf galaxies are often regarded as "fossil galaxies" formed in the early Universe, and their structures provide valuable clues to understanding how galaxies formed and evolved.
Traditionally, dwarf galaxies have been thought to form through relatively simple processes, such as gas inflow and outflow and internal star formation, meaning that galaxy–galaxy interactions or mergers were considered rare in such low-mass systems. However, recent observations by the European Space Agency’s Gaia mission have revealed that in some dwarf galaxies, stars are distributed beyond their expected outer boundary, known as the tidal radius. Because Gaia observations are limited to relatively bright stars, primarily red giant branch (RGB) stars, it has been difficult to investigate the detailed distribution using numerous faint stars in the outer regions. As a result, it has remained unclear whether these extended structures are the result of tidal interactions with the Milky Way or are intrinsic features formed through past galaxy mergers.
An international research team, led by the National Astronomical Observatory of Japan (NAOJ) and including SOKENDAI (The Graduate University for Advanced Studies), Hosei University, and Tohoku University, observed the Ursa Minor dwarf spheroidal galaxy (UMi dSph), a satellite galaxy of the Milky Way, using Hyper Suprime-Cam (HSC) on the Subaru Telescope. By combining one of the world’s widest fields of view, equivalent to nine full moons, with the powerful light-gathering capability of the 8.2-meter telescope, HSC enabled the team to investigate the faint stellar populations of the galaxy out to its outskirts beyond the nominal tidal radius. As a result, the team detected many faint main-sequence stars that were invisible to Gaia and successfully mapped the stellar distribution extending into the outskirts of the UMi dSph with unprecedented precision.
Their analysis reveals that the stellar distribution extends not only along the major axis, as previously known, but also along the minor axis (Figure 2). The structure along the minor axis shows properties distinct from the elongation along the major axis, which is commonly attributed to tidal forces from the Milky Way. This suggests that the minor-axis structure may have a different origin.
Figure 2: Spatial distribution of member main-sequence stars of the UMi dSph (central panel), and stellar number density profiles along the major and minor axes (left and right panels, respectively). The color map and contours in the central panel both represent the stellar surface density. The white dashed lines indicate the directions of the major and minor axes of the UMi dSph. The black curves in the side panels show the predicted number density profiles assuming no extended stellar structure. The observed number densities (blue and green points) exceed these predictions along both the major and minor axes, indicating the presence of an extended stellar structure in the outskirts. (Credit: NAOJ)
The minor-axis structure discovered around the UMi dSph may have been formed through a merger between dwarf galaxies. These findings suggests that galaxy interactions and mergers may have played a role in the formation and evolution of even extremely low-mass dwarf galaxies, with masses as small as one ten-thousandth that of the Milky Way.
Kyosuke Sato, the lead author of this study and a graduate student at SOKENDAI, says, "We have rarely found evidence of galaxy mergers in the Milky Way’s dwarf galaxies. This discovery offers a new way of thinking about how dwarf galaxies formed."
This study has revealed a previously hidden stellar structure in the outskirts of the UMi dSph, representing an important step toward understanding the formation and evolutionary history of dwarf galaxies. However, to determine whether this structure was formed by tidal interactions with the Milky Way or represents a remnant of a past merger, detailed studies of stellar kinematics and chemical abundances are required. Future observations with the Subaru Telescope’s new spectrograph, ʻŌnohiʻula PFS, are expected to reveal the origin of this structure.
This research has been published in The Astrophysical Journal Letters on October 23, 2025 (Sato et al., "The Extended Stellar Distribution in the Outskirts of the Ursa Minor Dwarf Spheroidal Galaxy").
This work was supported by JSPS KAKENHI (Grant Nos. JP18H05875, JP20K04031, JP20H05855, JP25K01047, and JP24K00669) and by JST SPRING, Japan (Grant No. JPMJSP2104). Part of this work was also supported by Oversea Travel Fund (2025) for students of the Astronomical Science Program, The Graduate University for Advanced Studies, SOKENDAI.
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.
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