The standard model of cosmology suggests that a halo of dark matter particles surrounds most galaxies. However, this halo of dark matter is invisible, but its mass exerts a strong gravitational pull on galaxies in the vicinity.
A new study by the University of Bonn (Germany) and the University of Saint Andrews (Scotland) challenges this Universe view. The results suggest that the dwarf galaxies of Earth’s second closest galaxy cluster—known as the Fornax Cluster—are free of such dark matter halos.
Scientists, in particular, have introduced an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational tides’ from nearby larger galaxies.
Elena Asencio, a Ph.D. student at the University of Bonn and the lead author of the story, said, “Tides arise when gravity from one body pulls differently on different parts of another body. These are similar to tides on Earth, which arise because the moon pulls more strongly on the side of Earth which faces the moon.”
Pavel Kroupa, professor at the University of Bonn and Charles University in Prague, said, “The Fornax Cluster has a rich population of dwarf galaxies. Recent observations show that some of these dwarfs appear distorted as if the cluster environment has perturbed them.”
“Such perturbations in the Fornax dwarfs are not expected according to the Standard Model. This is because, according to the standard model, the dark matter halos of these dwarfs should partly shield them from tides raised by the cluster.”
Based on internal characteristics and distance from the gravitationally strong cluster center, the authors calculated the expected level of disturbance of the dwarfs. Large yet low stellar mass galaxies and galaxies around the cluster core are more susceptible to disruption or destruction. They contrasted the outcomes with the level of disruption they had noticed in images collected by the European Southern Observatory’s VLT Survey Telescope.
Elena Asencio says, “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 cluster center even when the tides it raises on a dwarf are sixty-four times weaker than the dwarf’s self-gravity.”
“Not only is this counter-intuitive, but it also contradicts previous studies, which found that the external force needed to disturb a dwarf galaxy is about the same as the dwarf’s self-gravity.”
Based on this, scientists concluded that it is impossible to explain the observed morphologies of the Fornax dwarfs in a self-consistent way within the standard model.
They repeated the analysis using Milgromian dynamics (MOND). Instead of assuming dark matter halos surrounding galaxies, the MOND theory proposes a correction to Newtonian dynamics by which gravity experiences a boost in the regime of low accelerations.
Dr. Indranil Banik from the University of St Andrews said, “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. But our results show a remarkable agreement between observations and the MOND expectations for the level of disturbance of the Fornax dwarfs.”
Aku Venhola from the University of Oulu (Finland) and Steffen Mieske from the European Southern Observatory said, “It is exciting to see that the data we obtained with the VLT survey telescope allowed such a thorough test of cosmological models.”
Pavel Kroupa, a member of the transdisciplinary research areas, modeling, and matter at the University of Bonn, said, “This is not the first time that a study testing the effect of dark matter on the dynamics and evolution of galaxies concluded that observations are better explained when they are not surrounded by dark matter. The number of publications showing incompatibilities between observations and the dark matter paradigm keeps increasing every year. It is time to start investing more resources into more promising theories.”
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 which other teams should verify.”
- Elena Asencio, Indranil Banik et al. The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter. Monthly Notices of the Royal Astronomical Society, Volume 515, Issue 2, September 2022. DOI: 10.1093/mnras/stac1765
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