Andromedagalaxy is one of the next neighboring galaxies of our Milky Way. Like this, it is surrounded by an entire “court” of smaller dwarf galaxies. But the distribution of these Andromeda companions gives up astronomers. Because of the 37 dwarf galaxies of our neighbor, almost 80 percent frolic on only one side of Andromedagalaxy. This strong asymmetry can only be found in the universe in 0.3 percent of all comparable galaxy systems, as the astronomers found. In addition, the one -sided configuration through simulations of common galaxy interactions and galaxy developments cannot be replicated. How and why the dwarf galaxies of Andromedagalaxy are so unevenly distributed is still unclear. In any case, she makes our galactic neighbors a cosmological outlier.
The Andromedagalaxy M31 is only around 2.5 million light years away from the Milky Way. In the night sky she can already be seen with the naked eye as a weak, washed -out light spot. Andromedagalaxy has a comparable mass like our Milky Way, is also a spiral galaxy and has a similarly moving story of collisions and mergers. Another commonality are the numerous dwarf galaxies that circle the two larger galaxies. According to the Cosmological standard model, they are the remains of previous merger through which galaxies grow. Typically, the dwarf trabants would have to circle their massive taverns in an almost random arrangement.
Asymmetrically distributed dwarf galaxies
But this is not the case with Andromedagalaxy: “The dwarf galaxies observed around our neighboring galaxy Andromeda contradict this expectation, because they are highly arranged,” explain Kosuke Jamie Kanehisa from the Leibniz Institute for Astrophysics Potsdam (AIP) and his colleagues. Instead of being distributed around their host galaxy, the majority of these dwarf galaxies concentrate on one side of Andromedagalaxy. In order to examine this strange asymmetry more precisely, Kandehisa and his colleagues have analyzed a new data set for the positions and distances of 37 Trabant galaxies from Andromeda. This revealed: With just one exception, all Andromeda satellites are within a 107-degree cone that points to the Milky Way-a region that covers only 64 percent of the landlax. The fact that half of these satellite galaxies Andromeda circles Andromeda on the same level is even more unusual – similarly the sun is planned. The coexistence of such a level of satellite galaxies and a crooked satellite distribution is extremely unusual in the cosmological standard model, as the team explains.
This raises the question of whether Andromeda’s dwarf galaxies are an anomaly or whether our understanding of galaxy formation on small scales may be incomplete. In search of an answer, Kanehisa and his colleagues used simulations of galaxy development to determine possible causes and the frequency of such asymmetrical dwarf galaxies distributions. “With the help of two known simulations, we searched for Andromeda-like host galaxies and analyzed the spatial distribution of their dwarf satellites with the help of special metrics to quantify the asymmetry,” explains co-author Marcel Pawlowski from AIP. This showed that such asymmetrical distribution of dwarf galaxies is hardly reproducible even with similar galaxy couples such as Andromeda and Milky Way. “We have to look at more than three hundred simulated systems in order to find only one thing that is similarly extreme in its asymmetry as what observed,” reports Pawlowski. This makes Andromeda an extreme outlier who contradicts cosmological expectations. “Our analyzes show that such a pattern is extremely rare in current cosmological simulations,” says Kanehisa.
Cause unclear
Why Andromedagalaxy is so unusual in a cosmic comparison is still unclear. “The creation of this anisotropic structure around M31 represents a mystery that requires a unique evolutionary story to explain in view of its rarity among the simulated analogies,” the astronomers wrote. A possible explanation would be that the Andromedagalaxy, after astronomical standards, briefly acquired and bound a whole accumulation of these dwarf galaxies at once. It would also be conceivable that the gravity influence of the nearby Milky Way plays a role – even if this has already been taken into account in the simulations. “At the moment, no well -known educational mechanism can explain the collective asymmetry of the Andromeda system,” Kanehisa and his colleagues state. However, they hope to get more information about the causes of this configuration by further searching for galaxies with similarly asymmetrical dwarf galaxy distribution. Efforts to examine distant systems and search for comparable structures are already in progress and patterns of the next generation such as Euclid will accelerate this search.
Source: Kosuke Jamie Kanehisa (Leibniz Institute for Astrophysics Potsdam (AIP)) et al., Nature Astronomy, DOI: 10.1038/S41550-025-02480-3