Scientists have unearthed a groundbreaking discovery about dark matter halos that challenges our understanding of the Universe and provides an alternative gravity theory.
The Standard Model of cosmology suggests galaxies are predominantly surrounded by a halo of dark matter particles. These dark matter halos are invisible, but their mass emits a strong gravitational pull on nearby galaxies.
However, a new study led by the University of Bonn and the University of Saint Andrews provides an alternative view to this theory. Their investigation discovered that the dwarf galaxies of Earth’s second closest Galaxy cluster – the Fornax Cluster – do not have dark matter halos.
The Fornax Cluster
Dwarf galaxies are diminutive, faint galaxies usually found in Galaxy clusters or proximity to larger galaxies, meaning they may be affected by the gravitational effects of their larger neighbours.
Elena Asencio, a PhD student at the University of Bonn and the lead author of the research, commented: “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.”
Gravitational tides occur when gravity from one body pulls differently on different parts of another, similar to Earth’s tides caused by the Moon pulling more strongly on the side of Earth that faces it. The Fornax Cluster is rich in dwarf galaxies; however, recent observations show that some appear distorted as if the cluster environment had perturbed them.
Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague, said: “Such perturbations in the Fornax dwarfs are not expected according to the Standard Model. 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.”
New insights into dark matter halos
The researchers examined the expected level of disturbance of the dwarfs, which depends on their internal properties and distance to the gravitationally powerful centre of the cluster. Large galaxies with low stellar masses and galaxies close to the centre of the cluster are more easily disturbed or destroyed. The team compared these results with the observed level of disturbance from images obtained by the VLT Survey Telescope of the European Southern Observatory.
Asencio said: “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 cluster centre even when the tides it raises on a dwarf are sixty-four times weaker than the dwarf’s self-gravity.
“Not only is this counter-intuitive, but 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.”
The researchers stated that the Standard Model could not explain the morphologies observed in the dwarfs in a self-consistent way. Subsequently, the team performed the analysis again using Milgromian dynamics (MOND). Interestingly, instead of assuming dark matter halos surrounding galaxies, the MOND theory suggests a correction to Newtonian dynamics where gravity experiences a boost in the regime of low accelerations.
Dr Indranil Banik from the University of St Andrews explained: “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.”
This is not the first time a study analysing the effect of dark matter on the dynamics and evolution of galaxies has discovered the observations were better explained when they are not surrounded by dark matter.
Kroupa commented: “The number of publications showing incompatibilities between observations and the dark matter paradigm keeps increasing every year. It is time to start investing more resources into more promising theories.”
Dr Hongsheng Zhao from the University of St Andrews concluded: “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.”
