For years astronomers have worried about how to explain why the Milky Way has fewer small companion satellite galaxies than the standard dark matter model predicts. Now new observations from the Subaru Telescope have discovered new satellite galaxies, and raised the estimates for the total number expected to exist around the Milky Way. This solves the "Missing satellite problem," but now the estimates are too high for the standard dark matter model to explain.
Figure 1: The position of a newly found dwarf galaxy (Virgo III) in the constellation Virgo (left) and its member stars (right; those circled in white). The member stars are concentrated inside the dashed line in the right panel. Dwarf galaxies are diffuse and faint, having only a small number of stars, so in order to identify them, it is necessary to explore a large sky volume. HSC-SSP provides an ideal opportunity for this purpose. (Credit: NAOJ/Tohoku University)
How many small companion galaxies (satellite galaxies) does the Milky Way have? This has been an important question for astronomers for decades. This is because satellite galaxies are formed when cooled gas falls into small clumps of dark matter and stars are formed from the gas. Thus, satellite galaxies are related to the properties (the nature) of dark matter. According to the standard theory of dark matter (Note 1), galaxies like the Milky Way are predicted to have more than a thousand clumps of dark matter and the same number of satellite galaxies are expected. However, only a few dozen satellite galaxies have been found so far (Figure 2). This discrepancy in the number has been called the "Missing satellites problem." A rethink is needed to solve this problem, but it has been completely unclear what needs to be rethought. Maybe the true nature of dark matter is different from the standard theory, such that fewer clumps should be expected, or maybe star formation from gas in dark matter clumps is suppressed, making it difficult for satellite galaxies to form.
Figure 2: Satellite galaxies around the Milky Way Galaxy. The plane of the Galactic disk is on the horizontal plane. The blue squares are the Large and Small Magellanic Clouds, and the red circles are other satellite galaxies. The fainter their absolute visual magnitude, the smaller the dot size to indicate the satellite galaxy. The positions of Virgo III and Sextans II are indicated by arrows. (Credit: NAOJ/Tohoku University)
Another key to solving this problem is the possibility that there are many undiscovered faint satellite galaxies (dwarf galaxies) (Note 2), which are very far from our location in the Galaxy. To assess this possibility, the Subaru Telescope, with a large aperture of 8.2 meters, together with Hyper Suprime-Cam (HSC), the ultra-wide-field camera, form an extremely powerful combination for finding these faint dwarf galaxies. This is because, in order to discover such very faint galaxies, it is necessary to explore a wide field of view down to faint magnitudes, and the Subaru Telescope and HSC are the most powerful combination in the world for this purpose.
Figure 3: The area observed by the HSC Strategic Program (area surrounded by red lines). Previously known satellite galaxies are indicated by black squares, and newly discovered satellite galaxies are indicated by white triangles and stars. (Credit: NAOJ/Tohoku University)
The international research team searched for new dwarf galaxies using the large dataset obtained from the HSC Subaru Strategic Program (HSC-SSP), which covers a wide area of the sky (Figure 3). The HSC-SSP data have been sequentially analyzed and released, and the team has previously found three new dwarf galaxies, Virgo I, Cetus III, and Bootes IV. Now, the team has discovered an additional two new dwarf galaxies (Virgo III and Sextans II) from the latest data release (Figure 1). The team also found that all of these are located more than 300,000 light years away from the Sun.
In the HSC-SSP footprint (approximately 1,140 square degrees), four dwarf galaxies were previously known. With the discovery made by the research team, a total of nine satellite galaxies have been found. Although this may seem like a small number, it is actually much higher than the latest theoretical predictions.
The "Missing satellites problem" has led to theoretical research on the suppression process for the formation of dwarf galaxies. Against this background, the latest, most plausible analysis predicted that there should be about 220 satellite galaxies in total around the Milky Way. Among those 220, only about 3 to 5 satellite galaxies would be expected in this survey given the size of the HSC-SSP footprint and HSC’s detection limit. However, nine satellite galaxies have actually been identified, so if we convert this to the entire Milky Way volume, there will be at least 500 satellite galaxies. Now, we are faced with a "Too many satellites problem," rather than missing satellites.
This raises questions about the fundamental physical process of how stars form in a clump of dark matter, which is the size of a satellite galaxy. The current situation is the result of putting too much suppression on star formation, so astronomers need to reexamine it in detail, for example, to see if the calculations of the process are not accurate enough, or if there are physical processes that have not yet been considered. However, at least the "Missing satellites problem" that was initially challenging to the standard theory is close to being solved, and we can say that the standard model of dark matter is now in a position to survive.
Meanwhile, new observations over a wider area of the sky and down to fainter magnitudes, are needed in order to increase the statistical significance of the satellite galaxy count. One of these new observations is a large-scale photometric survey, the LSST (Legacy Survey of Space and Time), to be conducted by the Vera C. Rubin Observatory telescope, the Simonyi Survey Telescope, which is currently under construction. Starting next year, the survey will cover the entire sky that can be observed from the telescope's site in Chile. It is hoped that many new satellite galaxies will be discovered, which will solve all the problems surrounding dark matter and the galaxy formation process within it.
These results were published online as an advance article in the Publications of the Astronomical Society of Japan (PASJ) on June 8, 2024 (Homma et al., "Final Results of Search for New Milky Way Satellites in the Hyper Suprime-Cam Subaru Strategic Program Survey: Discovery of Two More Candidates").
This work is supported by Grants-in-Aid for Scientific Research (Grant Nos. JP18H05437 , JP21H05448 , JP24K00669 , JP20H01895 , JP21K13909 , and JP23H04009 ) .
(Note 1) In standard theory, dark matter is considered to be a group of elementary particles called cold dark matter.
(Note 2) Small, faint galaxies are called dwarf galaxies.
Research Team
Daisuke Homma (National Astronomical Observatory of Japan), Masashi Chiba (Tohoku University), Yutaka Komiyama (Hosei University), Masayuki Tanaka (National Astronomical Observatory of Japan), Sakurako Okamoto (National Astronomical Observatory of Japan), Mikito Tanaka (Hosei University), Miho N. Ishigaki (National Astronomical Observatory of Japan), Kohei Hayashi (Sendai National College of Technology), Nobuo Arimoto (formerly at the National Astronomical Observatory of Japan), Robert H. Lupton (Princeton University), Michael A. Strauss (Princeton University), Satoshi Miyazaki (National Astronomical Observatory of Japan) , Shiang-Yu Wang (Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan), and Hitoshi Murayama (Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo).
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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