No traces of dark matter halos

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

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 its mass exerts a strong gravitational pull on nearby galaxies. A new study conducted by the University of Bonn (Germany) and the University of Saint Andrews (Scotland) challenges this view of the Universe. The results suggest that dwarf galaxies in the second-closest cluster of galaxies to Earth, known as the Fornax cluster, are free of such dark matter halos. The study appeared in the journal Royal Astronomical Society Monthly Notices.

Dwarf galaxies are small, faint galaxies that are usually found in galaxy clusters or near larger galaxies. For this reason, they could be affected by the gravitational effects of their larger companions. “We are introducing an innovative way to test the Standard Model based on the amount of dwarf galaxies perturbed by gravitational tides from nearby larger galaxies,” said Ph.D. Elena Asencio. student at the University of Bonn and main author of the story. Tides occur when one body’s gravity pulls differently on different parts of another body. These are similar to tides on Earth, 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 had been disturbed by the environment of the cluster. “Such perturbations in Fornax dwarfs are not expected according to the Standard Model,” said Pavel Kroupa, a professor at the University of Bonn and Charles University in Prague. “Indeed, according to the standard model, the dark matter halos of these dwarfs should in part protect them from the tides raised by the cluster.”

The authors analyzed the expected level of disturbance of the dwarfs, which depends on their internal properties and their distance from the center of the gravitationally powerful cluster. Large but low stellar mass galaxies and galaxies close to the center of the cluster are more easily disrupted or destroyed. They compared the results with their observed level of disturbance evident from photographs taken by the European Southern Observatory’s VLT Survey Telescope.

Elena Asencio clarifies that “the comparison showed that, if one wants to explain the observations in the standard model. The dwarfs of Fornax should already be destroyed by gravity from the center of the cluster even when the tides that it raises on a dwarf are sixty-four times weaker”. than the dwarf’s own gravity.” Not only is this counterintuitive, she says, but it also contradicts previous studies, which found that the external force needed to disrupt a dwarf galaxy is roughly the same as the dwarf autogravity.

Contradiction with the standard model

From this, the authors concluded that, in the standard model, it is not possible to explain the observed morphologies of Fornax dwarfs in a self-consistent way. They repeated the analysis using Milgromian dynamics (MOND). Instead of assuming halos of dark matter surrounding galaxies, the MOND theory proposes a correction to Newtonian dynamics whereby gravity experiences an increase in the low acceleration regime.

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

“It is exciting to see that the data we have obtained with the VLT telescope has allowed such thorough testing of cosmological models,” said Aku Venhola from the University of Oulu (Finland) and Steffen Mieske from the European Observatory. austral, 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 best 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’s time to start investing more resources in more promising theories,” said Pavel Kroupa, member of the transdisciplinary research areas modeling and matter at the University. from Bonn.

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

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More information:
Elena Asencio et al, The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter, Royal Astronomical Society Monthly Notices (2022). DOI: 10.1093/mnras/stac1765

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