According to the standard model of cosmology, the vast majority of galaxies are surrounded by a halo of dark matter particles. This halo is invisible, but the mass exerts a strong gravitational pull on nearby galaxies. A new study led by the University of Bonn (Germany) and the University of Saint Andrews (Scotland) challenges this view of the universe. The results suggest that the dwarf galaxies of 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 in the vicinity of larger galaxies. Because of this, they can be affected by the gravitational effects of their larger companions. “We introduce an innovative way to test the standard model based on how much dwarf galaxies are perturbed by gravitational, tidal forces” from nearby larger galaxies, says Elena Asencio, a Ph.D. student at the University of Bonn and the main author of the story. Tides occur when the force of gravity from one body pulls differently on different parts of another body. These are similar to the tides on Earth, which occur because the Moon pulls more strongly on the side of the Earth facing 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 the cluster environment. “Such disturbances in the Fornax dwarfs are not expected according to the standard model,” said Pavel Kroupa, professor at the University of Bonn and Charles University in Prague. “This is because, according to the Standard Model, the dark matter halos of these dwarfs should partially shield them from the tides raised by the cluster.”
The authors analyzed the expected disturbance level of the dwarfs, which depends on their internal properties and their distance from the gravitationally powerful cluster center. Galaxies with large sizes but low stellar masses and galaxies near the cluster center are more easily disrupted or destroyed. They compared the results with the observed level of disturbance evident from photographs taken by the VLT Survey Telescope at the European Southern Observatory.
Elena Asencio says that “the comparison showed that if one wants to explain the observations in the Standard Model. Fornax dwarfs should already be destroyed by the gravity of the cluster center even when the tide it travels on a dwarf is sixty-four times weaker than the dwarf’s own gravity.” Not only is this counterintuitive, she said, it also contradicts previous studies, which found that the external force needed to disrupt a dwarf galaxy is about the same as the dwarf’s own gravity.
Opposition to the standard model
From this, the authors concluded that it is not possible in the standard model to explain the observed morphologies of the Fornax dwarfs in a self-consistent way. They repeated the analysis using Milgromian dynamics (MOND). Instead of assuming halos of dark matter surrounding galaxies, MOND theory proposes a correction to Newtonian dynamics where gravity experiences a boost in the regime of low accelerations.
“We were not sure that the dwarf galaxies would be able to survive the extreme environment of a galaxy cluster in MOND, due to the absence of protective dark matter halos in this model,” said Dr. Indranil Banik from the University of St. Andrews . “However, our results show a remarkable agreement between observations and the MOND expectations for the perturbation level of the Fornax dwarfs.”
“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 from the University of Oulu (Finland) and Steffen Mieske from the European Southern Observatory, co-authors of the study.
This is not the first time that a study testing the effect of dark matter on the dynamics and evolution of galaxies has concluded that observations are better explained when they are not surrounded by dark matter. “The number of publications showing incompatibility between observations and the dark matter paradigm only increases every year. It is time to start investing more resources in more promising theories,” said Pavel Kroupa, member of the interdisciplinary research areas modeling and matter at the University of Bonn .
Dr Hongsheng Zhao from 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 verify.”
New spin on galaxy rotation saves controversial gravity theory
Elena Asencio et al., The distribution and morphologies of Fornax Cluster dwarf galaxies suggest that they lack dark matter, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1765
Provided by the University of Bonn
Citation: No trace of dark matter halos (2022, August 5) retrieved August 6, 2022 from https://phys.org/news/2022-08-dark-halos.html
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