In a groundbreaking study, astronomers Cheng-Yu Chen and Chorng-Yuan Hwang reveal that six spiral galaxies exhibit a startling absence of dark matter. Using data from the MaNGA survey, the researchers analyzed rotation curves and baryonic mass, discovering that the galaxies’ dynamics can be fully explained by ordinary matter alone. This contradicts the widely accepted belief that dark matter is essential for galaxy formation and rotation stability. The study opens new discussions about alternative mechanisms in galaxy formation.
For decades, dark matter has been considered the invisible glue holding galaxies together, especially in spiral galaxies where the flatness of their rotation curves seemed impossible to explain without invoking this mysterious substance. Yet, new findings are shaking up this long-standing assumption. Astronomers Cheng-Yu Chen and Chorng-Yuan Hwang, in their 2024 study, have identified six spiral galaxies whose behavior defies the traditional dark matter model.
Using data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA), the researchers discovered that the rotation curves of these galaxies could be accounted for entirely by their baryonic, or ordinary, matter. In particular, the galaxies in question display a remarkably low ratio of dynamical mass (total mass inferred from gravitational effects) to baryonic mass, with values close to 1.09—much lower than typically expected in spiral galaxies where dark matter significantly outweighs visible matter.
This unexpected discovery follows from the team’s analysis of Hα velocity maps, which allowed them to precisely map the rotational dynamics of the galaxies. By applying a well-established relation between galaxy luminosity and rotation speed (the Tully-Fisher relation), they identified a group of galaxies that rotated far slower than expected. Among these, they selected the six most intriguing candidates where ordinary matter alone could explain the observed dynamics.
In contrast to prior studies, where dark-matter-deficient galaxies were usually thought to arise from unique or extreme formation conditions, these six galaxies appear unremarkable in many other respects. Their baryonic masses fall within the typical range for spiral galaxies (10^10 to 10^11 solar masses), and they reside in environments with varying densities of neighboring galaxies, suggesting that no special conditions led to their development.
The absence of significant dark matter in these galaxies presents a direct challenge to established theories of galaxy formation and rotation, which rely heavily on dark matter’s gravitational influence. It also calls into question the universality of Modified Newtonian Dynamics (MOND), an alternative to dark matter that adjusts the laws of gravity on galactic scales. In this case, the galaxies fall below the critical acceleration predicted by MOND, yet do not show the expected signs of dark matter.
To ensure the accuracy of their findings, Chen and Hwang carefully ruled out several alternative explanations. They confirmed that the galaxies were not viewed at such an angle that their rotation speeds could be underestimated. They also considered the possibility of extremely diffuse or extended dark matter halos but found no evidence for such structures.
The discovery of these dark-matter-deficient galaxies is not just a curiosity but has broader implications for our understanding of cosmic structures. While dark matter remains a cornerstone of the current cosmological model, this research suggests that some galaxies can form and evolve without relying on its influence. As the authors note, further studies using more extensive datasets, particularly in the radio spectrum to trace hydrogen gas, will be essential to confirm these findings and explore the mechanisms behind the formation of such galaxies.
Conclusion: This study opens the door to revaluating the role of dark matter in galaxy dynamics and formation. The findings could have profound implications for our understanding of the universe, suggesting that dark matter may not be as universally essential as once believed. Ongoing research will help determine whether these six galaxies are outliers or part of a broader trend, potentially reshaping our understanding of galaxy evolution.
Source: Six spiral galaxies lacking dark matter