The dwarf galaxy NGC1427A flies through the Fornax galaxy cluster and undergoes disturbances that would not be possible if this galaxy were surrounded by a heavy and extended dark matter halo, as required by standard cosmology.
Credit: ESO
Disturbances in the dwarf galaxies of one of Earth’s closest galaxy clusters point to a different gravity theory.
Dwarf galaxies are small, faint galaxies that are often found in or close to bigger galaxies or galaxy clusters. As a result, they could be impacted by their larger companions’ gravitational effects.
“We introduce an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational tides’ from nearby larger galaxies,” said Elena Asencio, a Ph.D. student at the University of Bonn and the lead author of the story.
Tides occur when gravity from one body pulls on various areas of another body differently. 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 seem distorted as if the cluster environment had perturbed them. “Such perturbations 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 partly shield them from tides raised by the cluster.”
The scientists examined the expected amount of disturbance of the dwarfs, which is determined by their internal properties and distance from the gravitationally powerful cluster center. Large galaxies with low stellar masses, as well as galaxies near the cluster center, are more easily perturbed or destroyed. They matched the findings 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 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 own self-gravity.” Not only is this counter-intuitive, she said, 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.
Contradiction to the standard model
From this, the authors concluded that, in the standard model, it is not possible 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 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.
“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 – admitted Dr. Indranil Banik from the University of St. Andrews – “but our results show a remarkable agreement between observations and the MOND expectations for the level of disturbance 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.
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