Distorted galaxies challenge the theory of gravity

An international team of astronomers recently studied the Fourneau cluster galaxies to test the ΛCDM (or Lambda CDM) model — the standard model that describes the Universe as homogeneous and isotropic, consisting of ordinary matter, dark matter and dark energy — and modified Newtonian dynamics (or MOND), a controversial alternative to general relativity theory. They identified disturbances within the cluster’s dwarf galaxies that suggest a different theory of gravity.

The majority of astronomers believe that the mysterious dark matter, which is the basis of the ΛCDM model, is the only way to account for certain phenomena, such as the mass and speed of galaxies, or simply their cohesion. This standard model of cosmology implies that most galaxies are surrounded by a halo of dark matter particles, capable of exerting a strong gravitational pull on nearby objects. Despite decades of research, the existence of dark matter has never been demonstrated and various alternative theories have been proposed.

One of them is the theory of modified Newtonian dynamics (or MOND theory). To test these different models, a team of astronomers took an interest in the dwarf galaxies of the Fourneau cluster, located some 62 million light-years from Earth. Dwarf galaxies are small, faint galaxies that are often located inside or near larger galaxies or galaxy clusters. Due to their low surface luminosity, they are particularly sensitive to tidal forces generated by more massive galaxies; they are therefore perfect objects of study to test the theories of gravity.

Dwarf galaxies without dark matter?

” We present an innovative way to test the Standard Model based on the extent to which dwarf galaxies are perturbed by “gravitational tides” from nearby large galaxies “, explains in a statement Elena Asencio, PhD student at the University of Bonn and first author of the study reporting the discovery. The degree of disturbance expected for these galaxies depends on the assumed law of gravity and the presence or absence of dark matter.

Observations suggest that several of the cluster’s dwarf galaxies had undergone severe deformations, as if the cluster’s environment had disturbed them. However, this is in total contradiction with the standard model, because in theory, the dark matter halos of dwarf galaxies should partly protect them from the tidal forces generated by the surrounding galactic cluster.

The team first looked at the expected level of disturbance from the dwarfs: this can be determined by their internal properties and their distance from the cluster’s powerful gravitational center. For example, large galaxies with low stellar mass, as well as galaxies close to the center of the cluster, are more easily disturbed or even destroyed. The researchers compared their results to the magnitude of the disturbances observed in images taken by the European Southern Observatory’s Very Large Telescope. This is where an inconsistency arose.

” The comparison showed that, if one wants to explain the observations by the standard model, the dwarfs of the Fourneau cluster should already be destroyed by the gravity of the center of the cluster, even when the tides it raises on a dwarf are 64 times weaker than the gravity of the dwarf itself! “, explains Elena Asencio. The scientist points out that this observation is not only counter-intuitive, but contradicts several studies which have shown that the external force necessary to disturb a dwarf galaxy is roughly equivalent to its own gravity.

The standard model repeatedly questioned

If these dwarf galaxies do not have dark matter halos, how did they come into existence? And above all, how were they able to resist against the surrounding forces of attraction? The standard model not being suitable to explain the observed phenomenon, the team undertook to apply another theory, the MOND theory, which is based on a modification of Newton’s second law (which relates the mass of an object and the acceleration it receives if forces are applied to it). It eliminates the presence of dark matter halos, but offers a correction whereby gravity is “boosted” at very low accelerations.

This pattern turned out to be much more consistent. ” Our results show a remarkable agreement between observations and MOND expectations for the level of disturbance of Furnace dwarfs said Dr. Indranil Banik, a researcher at the University of St. Andrews and co-author of the study.

The team points out that this is not the first time that a study testing the effect of dark matter on the dynamics and evolution of galaxies has challenged the theory. ” The number of publications showing incompatibilities between observations and the dark matter paradigm continues to increase every year. It’s time to start investing more resources in more promising theories said Dr. Pavel Kroupa, who heads the Stellar Populations and Stellar Dynamics research group at the University of Bonn.

These results obviously have major implications for fundamental physics. The team continues its investigations and expects to find other highly perturbed dwarf galaxies in other galactic clusters, inviting other research teams to explore this avenue.

Source : E. Asencio et al., Monthly Notices of the Royal Astronomical Society