No sign of dark matter halos

Dwarf galaxies are small, faint galaxies usually found in galaxy clusters or near larger galaxies. Because of this, they could be affected by the gravitational effects of their larger companions. “We present an innovative method for testing the Standard Model based on how much dwarf galaxies are perturbed by gravitational ‘tides’ from nearby larger galaxies,” said Elena Asencio, a PhD student at the University of Bonn and the story’s lead author. Tides are formed when the gravity of one body acts differently on different parts of another body. These are similar to the tides on Earth that occur because the moon pulls harder on the moon-facing side of the earth.

The Fornax Cluster has a rich population of dwarf galaxies. Recent observations show that some of these dwarfs appear distorted, as if disturbed by the cluster’s environment. “Such disturbances in the fornax dwarfs are not to be expected according to the standard model,” says Pavel Kroupa, professor at the University of Bonn and Charles University in Prague. Dwarfs should partially protect them from the tides created by the cluster.”

The authors analyzed the expected degree of disturbance of the dwarfs, which depends on their internal properties and their distance from the gravitational center of the cluster. Galaxies with large sizes but small stellar masses and galaxies near the center of the cluster are more easily perturbed or destroyed. They compared the results to their observed degree of disturbance, which is evident from photos taken by the European Southern Observatory’s VLT Survey Telescope.

“The comparison showed that if one wants to explain the observations in the standard model” – said Elena Asencio – “the fornax dwarfs should already be destroyed by gravity from the cluster center, even if the tides that lift them to a dwarf sixty-four times weaker than the dwarf’s own gravity.” Not only is this counterintuitive, she said, it contradicts previous studies that found that the external force required to perturb a dwarf galaxy is roughly equal to the dwarf’s own gravity .

Contradiction to the standard model

The authors concluded from this that it is not possible to explain the observed morphologies of the fornax dwarfs in a consistent manner using the standard model. They repeated the analysis using Milgromian dynamics (MOND). Instead of assuming dark matter halos surrounding galaxies, the MOND theory proposes a Newtonian dynamics correction that gives gravity a boost in the low-acceleration region.

“We weren’t sure if the dwarf galaxies would be able to survive the extreme environment of a galaxy cluster in MOND, since there are no protective dark matter halos in this model – said Dr. Indranil Banik of the University of St Andrews – “but our results show remarkable agreement between the observations and the MOND expectations for the magnitude of the disruption of the fornax dwarfs”.

“It is exciting to see that the data we obtained with the VLT survey telescope enabled 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 can be better explained when they are not surrounded by dark matter. “The number of publications showing incompatibilities between observations and the dark matter paradigm is increasing every year. It’s time to invest more resources in more promising theories,” said Pavel Kroupa, a member of the transdisciplinary research areas “Modeling” and “Matter”. ‘ at the University of Bonn.

dr Hongsheng Zhao from the University of St Andrews added: “Our results have major implications for fundamental physics. We expect to find more disturbed dwarves in other clusters, a prediction for other teams to check.”

Participating institutions and funding:

In addition to the University of Bonn, the University of Saint Andrews (Scotland), the European Southern Observatory (ESO), the University of Oulu (Finland) and Charles University in Prague (Czech Republic) were involved in the study. The study was supported by the University of Bonn, the UK Science and Technology Facilities Council and the German Academic Exchange Service.

Publication: Elena Asencio, Indranil Banik, Steffen Mieske, Aku Venhola, Pavel Kroupa & Hongsheng Zhao: The distribution and morphologies of the Fornax cluster dwarf galaxies suggest that they lack dark matter. Monthly Bulletins of the Royal Astronomical Society; https://doi.org/10.1093/mnras/stac1765 / https://arxiv.org/abs/2208.02265