Dwarf galaxies of one of Earth’s nearest clusters lack dark matter, new study suggests
Galaxies are thought to form inside immense halos of dark matter - an invisible form of matter that makes up most of the universe's mass. However, a new study has found that dwarf galaxies of one of Earth's nearest clusters lack this dark matter.
According to the standard model of cosmology, the vast majority of galaxies are surrounded by a halo of dark matter particles. By showing that the dwarf galaxies - small, faint galaxies - of Earth's second closest galaxy cluster - named the Fornax Cluster - are free of such dark matter halos, the new study has challenged this view. The Fornax Cluster has a rich population of dwarf galaxies, with recent observations showing that some of these dwarfs appear distorted.
Dwarf galaxies can usually be found in galaxy clusters or near larger galaxies, because of which, they might be affected by the gravitational effects of their larger companions.
Led by Elena Asencio, a PhD student at the University of Bonn, the study introduced an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational tides from nearby larger galaxies.
"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," Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague, said in a statement.
The researchers analysed the expected level of disturbance of the dwarfs, which depends on their internal properties and their distance to the gravitationally powerful cluster centre and compared this with their observed level of disturbance evident from photographs. They found 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.
The team concluded that the standard model cannot explain the observed morphologies of the Fornax dwarfs and the lack of fragile dwarfs towards its centre. They further 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.
"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," said Dr Indranil Banik of the School of Physics and Astronomy at St Andrews, who did much of the coding for this project.
The findings are published in Monthly Notices of the Royal Astronomical Society.