Do Even Low-mass Dwarf Galaxies Merge? New Clues from the Outer Stars of a Milky Way Satellite

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)

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)

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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.

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.

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.

Source: Subaru Telescope

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About the Subaru Telescope

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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The Ghostly Remnants of Galaxy Interactions Uncovered in a Nearby Galaxy Group

Figure 1: Pseudo-color images from HSC observation which contains M81, M82, and NGC 3077. Diameter of the FOV is 1.5 degrees. Bottom-left: close-up of M81. Bottom-center: further close-up of M81 showing the spiral arm. Bottom-right: color composite of the images used for the analysis. Click to enlarge each frame. (Credit: NAOJ/HSC Project)

Movie: Neighborhood of the spiral galaxy M81 

Credit: NAOJ/HSC Project)

Cosmological archaeological studies such as this one help astronomers refine their understanding of galaxy formation and evolution. The currently favored cosmological galaxy models are based on the idea of hierarchical structure formation: that structures in the universe such as galaxies develop from small "overdensities" to become large-scale objects. For example, the Milky Way and M81 first formed as part of a local over-density in the primordial matter distribution – that is, the earliest accumulations of matter in the young universe. They grew over time via the agglomeration of numerous smaller building blocks, some of which may have survived later mergers to become present-day dwarf satellite galaxies. Establishing the presence and nature of these satellites, and determining the large-scale structure and stellar content of halos in spiral galaxies, is essential to understand and explain the physics of hierarchical galaxy assembly.

Over the last decade, astronomers doing large photometric surveys (that is, measuring the light intensities of celestial objects) have found a number of new satellite galaxies, stellar streams, and over-densities around the Milky Way and the Andromeda galaxies. The detailed properties of stars in these systems are studied to reconstruct the stellar contents of galaxies in the early stage, which is called "Galactic Archeology" or "near-field cosmology". For the Galactic Archeology study, it is necessary to resolve individual stars in a galaxy, and observe across a good fraction of the galaxy's radius.

Until now, the outskirts of the Milky Way and Andromeda are the only places that have been surveyed to sufficiently faint depths to enable detailed tests of hierarchical galaxy assembly process across wide scales.

The observing team started the M81 archeology study by using Hyper Suprime-Cam (HSC). M81, also known as Bode's Galaxy, is located at a distance of 11.7 million light-years, and is one of the nearest massive spiral galaxies similar to the Milky Way. The super-wide field of view of the HSC allowed the team to observe out to a projected radius of a half-million light-years from the center of M81. The field includes 18 known member galaxies of the M81 group in only seven pointings. The camera's high sensitivity enabled the team to observe vast numbers of old red giant branch (RGB) stars as well as young main-sequence (MS) stars, red supergiants, and asymptotic giant-branch stars at the distance of M81.

The left panel in Figure 2 shows the spatial distribution of young MS stars and core helium-burning stars, which are color-coded according to their i-band luminosity. Bright stars are mainly located in the inner disk of M81, while most of the young stars in outlying concentrations are fainter than i=24 mag and have similar luminosity distributions as that of the stellar stream between M81 and NGC 3077. They are between 30-160 million years old. The study indicates the ages of stars in these tidal features are synchronized to each other, and that these systems were produced by recent tidal interactions between M81, M82 and NGC 3077.

Figure 2: Young main-sequence (MS) stars and red-giant branch (RGB) stars around M81, M82, and NGC 3077. Left: yellow is brighter stars, and blue is fainter stars. Right: color-coded for the metallicity, namely yellow is metal rich, blue is metal poor. Solid line shows the R25 radius of the galaxy measured in the visible light. (Credit: NAOJ)

The distribution of RGB stars in the right panel of Figure 2 shows that the extended stellar halos of the three main galaxies overlap each other, and that the outer regions of M82 and NGC 3077 are highly perturbed. This is likely a consequence of the recent gravitational encounter.

The color of each point in the figure is a rough proxy for metallicity. The RGB stars in M82's outer halo have significantly bluer colors, showing that they are more metal-poor than those in M81, the NGC 3077 halos and the inner halo of M82. The satellite galaxies, KDG 61, BK5N, and IKN cannot be seen in the maps of young stars, but appear as over-densities of old populations in the right panel. 

This implies they are not the product of the recent interaction between M81, M82 and NGC 3077.

The science team for this study consists of astronomers at Shanghai Astronomical Observatory, National Astronomical Observatory of Japan, Hiroshima University, University of Edinburgh, and University of Cambridge. Their first results from the M81 study with Suprime-Cam on Subaru Telescope were released in March 2010 at: (http://www.subarutelescope.org/Pressrelease/2010/03/18/index.html).

Team member Dr. Sakurako Okamoto (Shanghai Astronomical Observatory) commented on this program: "Our deep panoramic view of the M81 group demonstrates that the complexity long known to be present in neutral hydrogen (HI) is equally matched in the low surface brightness stellar component. Together with the Galactic Archeology study based on the HSC wide-field survey of the Subaru Strategic Program, we hope to establish the presence and nature of satellite galaxies, and determine the large-scale structure and stellar content of halos of spiral galaxies in general".

The team members are grateful to the entire staff at Subaru Telescope and the HSC team. They acknowledge the importance of Maunakea within the indigenous Hawaiian community.

The research paper titled "A Hyper Suprime-Cam View of the Interacting Galaxies of the M81 Group" will be published in the Astrophysical Journal Letters. This work was supported by the grants of CAS (XDB09010100), NSFC (11333003), and JSPS (Grant-in-Aid for Young Scientists B, 26800103).

Members of the research team:

• Sakurako Okamoto: Shanghai Astronomical Observatory, China
• Nobuo Arimoto: Subaru Telescope, National Astronomical Observatory of Japan/SOKENDAI (The Graduate University for Advanced Stuties), Japan
• Yoshihiko Yamada: Subaru Telescope, National Astronomical Observatory of Japan
• Yosuke Utsumi: Hiroshima Astrophysical Science Center, Hiroshima University, Japan
• Annette Ferguson: Institute for Astronomy, University of Edinburgh, Royal Observatory, UK
• Edouard Bernard: Institute for Astronomy, University of Edinburgh, Royal Observatory, UK
• Mike Irwin: Institute of Astronomy, University of Cambridge, UK

Source: Subaru Telescope