According to cosmology, a halo of dark matter particles surrounds the vast majority of galaxies. As a result, galaxies in the neighborhood are strongly gravitationally pulled by this invisible halo.
This view of the Universe has been challenged by a new study from the University of Bonn and the University of Saint Andrews (Scotland). According to these results, Earth’s second closest galaxy cluster, the Fornax Cluster, does not contain dark matter halos. An article describing the study appeared in Monthly Notices of the Royal Astronomical Society.
It is common to find dwarf galaxies in galaxy clusters or near larger galaxies that are small. These galaxies are faint and associated with more massive galaxies.
Due to this, their larger companions might exert gravitational effects on them.
“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 study.
Tides result when one body’s gravity pulls on another in different ways. As on Earth, these tides occur as the moon pulls more strongly on Earth’s side facing the moon.
Dwarf galaxies are abundant in the Fornax Cluster. However, evidence shows that some of these dwarfs have been perturbed by the cluster’s environment, causing them to appear distorted.
“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.
Apparently, this is due to the fact that, according to the standard model, these dwarfs should partly be shielded from the tides generated by the cluster.
Based on the distance the dwarfs are from the gravitationally powerful cluster center and their internal properties, the authors measured the level of disturbance the dwarfs will experience.
Massive galaxies with low stellar masses and galaxies near cluster centers are more prone to disruption or destruction.
Their results were compared to photos taken by the VLT Survey Telescope of the European Southern Observatory that showed a similar level of disturbance.
“The comparison showed that, if one wants to explain the observations in the standard model,” explained 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,” she added.
This is not only counterintuitive but also contradicts previous studies, which found that a dwarf galaxy’s self-gravity was enough to disturb it.
This led the authors to conclude that the standard model cannot account for the observed morphologies of the Fornax dwarfs.
This analysis was repeated using Milgromian dynamics (MOND).
Instead of assuming dark matter halos surrounding galaxies, MOND proposes a method of correcting Newtonian dynamics so that gravity boosts in regions of low acceleration.
“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, explained 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.”
Aku Venhola from the University of Oulu (Finland) and Steffen Mieske from the Galaxy Cluster Near Earth Shows Signs Of Disturbance, co-authors of the study, said it is exciting to see that the data we obtained with the VLT survey telescope allowed such a thorough test of cosmological models.
There have been several studies testing the effect of dark matter on galaxies’ dynamics and evolution that have concluded that observations are more easily explained when they are not surrounded by dark matter.
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.”
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