Signs of disturbance in the dwarf galaxies of one of Earth's nearest clusters indicate they lack dark matter and are better explained by an alternative gravity theory, according to a new study by an international group of astronomers, including physicists at the University of St Andrews.
According to the standard model of cosmology, the vast majority of galaxies are surrounded by a halo of dark matter particles. The new study challenges this view of the universe by showing 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 usually be found in galaxy clusters or near larger galaxies. Because of this, they might be affected by the gravitational effects of their larger companions.
The study, led by Elena Asencio, a PhD student at the University of Bonn, introduced an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational 'tides' from nearby larger galaxies. 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.
The Fornax Cluster has a rich population of dwarf galaxies. Recent observations show that some of these dwarfs appear distorted, as if they have been perturbed by the cluster environment.
Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague, said: "It was not expected that such perturbations should be present in the Fornax dwarfs. 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 authors analysed the expected level of disturbance of the dwarfs, which depends on their internal properties and their distance to the gravitationally powerful cluster centre. This was compared with their observed level of disturbance evident from photographs. The comparison showed that, if one wants to explain the observations in the standard model, the Fornax dwarfs should already be destroyed by gravity from the Fornax Cluster even when the tides it raises on a dwarf are 64 times weaker than the dwarf's own self-gravity.
Elena Asencio said: "Not only is this counter-intuitive, 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."
From this, the authors concluded that the standard model cannot explain the observed morphologies of the Fornax dwarfs and the lack of fragile dwarfs towards its centre.
However, the researchers were not content to leave the case of the Fornax dwarfs unsolved. They repeated the analysis using Milgromian dynamics (MOND). Instead of assuming dark matter halos surrounding galaxies, MOND proposes a correction to Newtonian dynamics by which gravity experiences a boost in the regime of low accelerations.
Dr Indranil Banik of the School of Physics and Astronomy at St Andrews, who did much of the coding for this project, says: "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. But our results show a remarkable agreement between observations and the MOND expectations for the level of disturbance of the Fornax dwarfs."
Dr Aku Venhola and Dr Steffen Mieske, 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."
This is not the first time that a study testing the effect of dark matter on the dynamics and evolution of galaxies concluded that observations are better explained when they are not surrounded by dark matter. A recent detailed review of the available evidence strongly prefers MOND.
Of the broader context of the Fornax results, Professor Kroupa said: "The number of publications showing incompatibilities between observations and the dark matter paradigm just keeps increasing every year. It is time to start investing more resources into more promising theories."
Dr Hongsheng Zhao, of the School of Physics and Astronomy at St Andrews, adds: "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."
