Dwarf Galaxy NGC1427A

New discovery points to an alternative theory of gravity

Dwarf galaxy NGC1427A

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

Perturbations in dwarf galaxies in one of the galaxy clusters closest to Earth point to a different theory of gravity.

Dwarf galaxies are small, faint galaxies that are often found in or near larger galaxies or galaxy clusters. As a result, they could be impacted 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 areas of another body. These are comparable to tides on Earth, which form when the moon exerts a stronger pull on the side of the Earth that faces the moon.

The Fornax cluster is home to a rich population of dwarf galaxies. Recent observations suggest that several of these dwarfs appear distorted as if the cluster environment had disturbed them. “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 scientists looked at the expected amount of disturbance from the dwarfs, which is determined by their internal properties and their distance from the center of the gravitationally powerful cluster. Large galaxies with low stellar mass, as well as galaxies close to the center of the cluster, are more easily disrupted or destroyed. They compared the results to the amount of disturbance shown in 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 dwarfs of Fornax should already be destroyed by gravity from the center of the cluster even when the tides that he lifts 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 about the same as the dwarf’s self-gravity.

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 – admitted Dr. Indranil Banik of the University of St. Andrews – “but our results show remarkable agreement between observations and MOND expectations for the level of disturbance 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 continues to increase each year. It’s time to start investing more resources in more promising theories,” said Pavel Kroupa, member of the transdisciplinary research areas “Modelling” 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 perturbed dwarfs in other clusters, a prediction that other teams should verify.

Reference: “The distribution and morphologies of the dwarf galaxies of the Fornax group suggest that they lack dark matter” by Elena Asencio, Indranil Banik, Steffen Mieske, Aku Venhola, Pavel Kroupa and Hongsheng Zhao, June 25, 2022, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stac1765

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