Dwarf Galaxies in a Cluster Near to Earth Appear to Be Devoid of Dark Matter

When you ask an astronomer about dark matter, they will always mention about how the cosmos is filled with this enigmatic, unseen stuff. It is specifically found in the halos that encircle the majority of galaxies. The galaxy itself, as well as other galaxies nearby, are strongly gravitationally influenced by the mass of the halo. That has been the accepted theory on dark matter and how it affects galaxies. The concept of such halos is not without flaws, though. Evidently, there are certain weirdly shaped dwarf galaxies that appear to lack halos.

How is that feasible, then? Do they present a challenge to the observed dark matter halo hypotheses that are currently held?

Galaxies are shielded by dark matter halos or shells from the gravitational pull of their close galactic neighbours according to the so-called “standard model” of cosmology. This view is now being challenged in a study led by the University of Bonn and the University of Saint Andrews (Scotland).

The results suggest that these dark matter halos are absent from the dwarf galaxies in the Fornax Cluster, the second closest galaxy cluster to Earth. The findings were published in the Monthly Notices of the Royal Astronomical Society.

Elena Asencio, a PhD candidate at the University of Bonn and the principal author of the study, said, “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.”

Small, faint galaxies known as dwarfs are generally located in or close proximity to larger galaxies or galaxy clusters. They might therefore be affected by the gravitational effects of their larger neighbours. Recent research reveals that some of these dwarfs have distorted appearances, as though the cluster environment has disturbed them.

The Standard Model does not predict “such perturbations in the Fornax dwarfs,” said Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague, and added that it’s because dark matter halos of these dwarfs should partially protect them from tides brought on by the cluster, according to the Standard Model.

Based on internal characteristics and distance from the gravitationally strong cluster centre, the authors calculated the expected level of disturbance of the dwarfs.

According to Asencio, the comparison revealed “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 64 times weaker than the dwarf’s own self-gravity.”

This goes against the findings of earlier research that the amount of force required to perturb a dwarf galaxy is roughly equal to the dwarf’s own gravity.

The authors deduced from this that the observed morphologies of the Fornax dwarfs cannot be self-consistently explained by the mainstream paradigm. Dr Hongsheng Zhao from the University of St Andrews said that their findings have significant ramifications for fundamental physics, and that they expect to find additional perturbed dwarfs in other clusters.