There is no trace of dark matter halos

There is no trace of dark matter halos

The dwarf galaxy NGC1427A flies through the Fornax galaxy cluster and experiences perturbations that would not be possible if this galaxy were surrounded by a heavy and extended dark matter halo, as required by standard cosmology. Credit: ESO

According to the Standard Model of cosmology, most galaxies are surrounded by a halo of dark matter particles. This halo is invisible, but its mass exerts a strong gravitational pull on nearby galaxies. A new study led by the University of Bonn (Germany) and the University of St. Andrews (Scotland) challenges this view of the universe. The results suggest that dwarf galaxies in the second closest galaxy cluster to Earth – known as the Fornax Cluster – are free of such dark matter halos. The study appeared in the journal Monthly Notices of the Royal Astronomical Society.

Dwarf galaxies are small, faint galaxies that can usually be found in galaxy clusters or near larger galaxies. Because of this, they can be affected by the gravitational effects of their larger companions. “We are introducing an innovative way to test the standard model based on how much dwarf galaxies are perturbed by gravitational ‘tides’ from nearby larger galaxies,” said Elena Asensio, a PhD student at the University of Bonn and lead author of the story. Tides occur when gravity from one body pulls differently on different parts of another body. They are similar to Earth’s tides, which occur because the moon pulls more strongly on the side of the Earth that faces the moon.

The Fornax cluster has a rich population of dwarf galaxies. Recent observations show that some of these dwarfs appear distorted, as if they have been disturbed by cluster environment. “Such perturbations in Fornax dwarfs are not expected according to the Standard Model,” said Pavel Krupa, a professor at the University of Bonn and Charles University in Prague. “This is because according to the standard modelsaid dark matter halos of these dwarfs should partially shield them from the ebb and flow caused by the cluster.”

The authors analyzed the expected level of perturbation of the dwarfs, which depends on their internal properties and their distance to the gravitationally powerful center of the bowl. Galaxies with large sizes but low stellar masses and galaxies near the cluster center are more easily disrupted or destroyed. They compared the results to the level of disturbance they observed, as seen in images taken by the European Southern Observatory’s VLT Survey Telescope.

Elena Asensio says that “the comparison showed that if one wants to explain the observations in the standard model, the Fornax dwarfs should already be destroyed by gravity from the center of the cluster, even when the tides it induces on the dwarf are sixty-four times stronger weaker than the dwarf’s own gravity.” This is not only counterintuitive, she said, but also contradicts previous studies that have found that the external force required to disturb a dwarf galaxy is about the same as the dwarf’s own gravity.

A contradiction of the standard model

From this, the authors conclude that it is not possible in the Standard Model to explain the observed morphologies of Fornax dwarfs in a self-consistent way. They repeated the analysis using Milgrom dynamics (MOND). Instead of assuming dark matter halos around galaxies, MOND theory proposes a correction to Newtonian dynamics whereby gravity experiences a boost in the regime of low accelerations.

“We weren’t sure that dwarf galaxies will be able to survive in the extreme environment of a galaxy cluster in MOND, due to the lack of protective dark matter halos in this model,” said Dr Indranil Banik of the University of St Andrews. “But our results show a remarkable agreement between observations and MOND expectations about the anxiety level of the Fornax dwarves.”

“It is exciting to see that the data we obtained with the VLT survey telescope allowed such a thorough test of cosmological models,” said Aku Venhola of the University of Oulu (Finland) and Steffen Mieske of the European Southern Observatory, co-authors of the study.

This is not the first time that research has tested the effect of dark matter on the dynamics and evolution of galaxies concluded that the observations are better explained when they are not surrounded by dark matter. “The number of publications showing inconsistency between observations and the dark matter paradigm just keeps increasing every year. It’s time to start investing more resources in more promising theories,” said Pavel Krupa, Member of the Transdisciplinary Research Areas, Modeling and matter at the University of Bonn.

Dr Hongsheng Zhao of the University of St Andrews added that their “results have major implications for fundamental physics. We expect to find more disturbed dwarfs in other clusters, a prediction that other teams should check.”

A new spin on the galaxy’s spin saves the controversial theory of gravity

More info:
Elena Asencio et al, Distribution and morphology of Fornax cluster dwarf galaxies suggest they lack dark matter, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1765

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University of Bonn

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