According to the standard cosmological model, the vast majority of galaxies are surrounded by a halo of dark matter particles. This halo is invisible, but its mass has a strong gravitational pull on nearby galaxies. A new study calls this Universe view into question. The findings imply that the dwarf galaxies of Earth’s second closest galaxy cluster, known as the Fornax Cluster, are free of such dark matter halos.
Dwarf galaxies are small, faint galaxies that can be found in galaxy clusters or near larger galaxies. As a result, they may be affected by the gravitational effects of their larger companions.
“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 PhD student at the University of Bonn and the lead author of the story. 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.
Dwarf galaxies abound in the Fornax Cluster. Recent observations show that some of these dwarfs appear distorted, as if they have been perturbed by the cluster environment. “Such perturbations in the Fornax dwarfs are not expected according to the Standard Model,” said Pavel Kroupa, Professor at the Universities of Bonn and Charles University in Prague. “This is because, according to the standard model, the dark matter halos of these dwarfs should partially shield them from the cluster’s tides.”
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. This is because, according to the standard model, the dark matter halos of these dwarfs should partially shield them from the cluster’s tides.
Elena Asencio
The authors analyzed the expected level of disturbance of the dwarfs, which depends on their internal properties and their distance to the gravitationally powerful cluster center. Galaxies with large sizes but low stellar masses and galaxies close to the cluster centre are more easily disturbed or destroyed. They compared the results with their observed level of disturbance evident from photographs taken by the VLT Survey Telescope of the European Southern Observatory.
“The comparison revealed that, if the standard model is used to explain the observations,” 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.” This is not only counter-intuitive, but it also contradicts previous research, which found that the external force required to disturb a dwarf galaxy is roughly equal to the dwarf’s self-gravity.
Contradiction to the standard model
The authors concluded that the observed morphologies of the Fornax dwarfs cannot be explained in a self-consistent manner using the standard model. They repeated the analysis with Milgromian dynamics (MOND). Instead of assuming dark matter halos surrounding galaxies, the MOND theory proposes a correction to Newtonian dynamics in 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.
This is not the first time that a study examining how dark matter affects the dynamics and evolution of galaxies came to the conclusion that observations are better explained when the galaxies are not surrounded by dark matter. “Every year, there are more publications that demonstrate how observations and the dark matter paradigm are incompatible. According to Pavel Kroupa, a member of the transdisciplinary research areas “Modeling” and “Matter,” it is time to start putting more money into more promising theories” in Bonn at the university.
Dr. Hongsheng Zhao from the University of St Andrews added: “Our results have major implications for fundamental physics. We expect to find more disturbed dwarfs in other clusters, a prediction which other teams should verify.”
