These dwarf galaxies seem devoid of dark matter, and there’s no point

Ask astronomers about dark matter and one of the things they talk about is that this invisible, mysterious ‘stuff’ pervades the universe. Notably, it is present in the halo surrounding most galaxies.

The mass of the halo exerts a strong gravitational effect on the Milky Way as well as others in the vicinity. This is largely the standard view of dark matter and its effects on galaxies.

However, there are problems with the idea of ​​those auras. Apparently, there exist some strangely shaped dwarf galaxies that look like they have no halo. How is this possible? Do they represent an observationally inspired challenge to prevailing ideas about dark matter halos?

Detecting troubled dwarf galaxies

In the so-called “Standard Model” of cosmology, dark matter spheres or halos protect galaxies from the gravitational effects of nearby galactic neighbors.

However, when astronomers from the University of Bonn and St Andrews in Scotland looked into the nearby Fornax cluster, which is about 62 million light-years away from us, they noticed something strange.

It contains many dwarf galaxies with distorted, distorted shapes. This is strange, especially if they must be surrounded by dark matter halos.

These dwarf galaxies seem devoid of dark matter, and there's no pointFornax Galaxy Cluster. (ESO/Jay Emerson/Vista)

Let’s take a quick look at dwarf galaxies. They are small and faint and are commonly found in galaxy clusters or riding with much larger companions. The Milky Way is a circle of dwarf galaxies surrounding the Galaxy.

In fact, it is a cannibal like the Sagittarius Dwarf Spheroid. Interestingly, recent studies suggest that at least one of the dwarf galaxies we have, called Tucana II, has a surprisingly massive dark matter halo.

So, what’s happening in Fornax that’s different?

There, dwarf galaxies can be “perturbed” by gravitational tides from nearby larger ones in the cluster. Tides occur when gravity from one body exerts different pulls on different parts of the other body. These are similar to tides on Earth when the Moon pulls more strongly on the side of the Earth that faces it.

The distorted shapes of dwarf galaxies observed by the team indicate a problem in our understanding of dark matter.

“Such disturbances in fornax dwarfs are not expected according to the Standard Model,” said Professor Pavel Krupa from the University of Bonn and Charles University in Prague.

“That’s because, according to that model, the dark matter halos of these dwarfs should partially shield themselves from the tides picked up by the cluster.”

Distorted dwarf galaxies explained

Krupa and Ph.D. student Elena Asencio analyzed observations of troubled dwarfs in the fornax. They wanted to understand the extent to which these galaxies exhibit gravitational distortions and what causes them.

The expected level of distortion depends on a few factors. One is the intrinsic characteristics of a dwarf galaxy. In addition, their distance from the center of the cluster is important. Whereas the gravitational effect is stronger.

As a rule, galaxies with large sizes but not many stars can be easily disturbed by strong gravitational tides. The same is true for galaxies closer to the center of the cluster.

Team members compared what they saw in the cluster with observations made by the VLT Survey Telescope at the European Southern Observatory. Asencio explained that what they found pointed to problems with the Standard Model.

“The comparison showed that, if one wants to interpret the observations in the Standard Model,” she said, “Fornax dwarfs must have already been destroyed by gravity from the cluster center, even if the tide rises on a dwarf. Sixty-four times weaker than the dwarf’s own self-gravity.”

Not only is this counter-intuitive, she said, it also contradicts previous studies. The team also found that the force required to perturb a dwarf galaxy is about the same as its self-gravity.

What does this mean for the Standard Model?

The research team explains that these puzzling, puzzling shapes of dwarf galaxies in the Fornax are difficult to explain if they are surrounded by dark matter. In other words, they shouldn’t be mistaken if they have a halo.

Still, there they are, with disturbing-looking figures. This means that there is no dark matter around those galaxies.

Clearly, if what astronomers have found is confirmed, the Standard Model needs some tweaking. And, there is at least one alternative explanation for the strange galaxy shapes. This is called the MOND model (short for Modified Newtonian Dynamics).

This suggests that Newton’s law of universal gravitation should be modified to take into account the observed properties of galaxies. This can be applied to explain why misshapen galaxies look the way they do.

According to Hongsheng Zhao, a member of the research team at the University of St Andrews, finding disturbed dwarfs without dark matter halos is a major challenge to the current approach.

It states that galaxies have halos. It appears that not all of them do, he explains.

“Our results have major implications for fundamental physics,” he said. “We expect to find more troubled dwarfs in other groups, a prediction that other teams must verify”.

This article was originally published by Universe Today. Read the original article.


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