Dwarf galaxies unaffected by dark matter discovered

Ask astronomers about dark matter and one of the things they tell you is that this invisible, mysterious “stuff” pervades the universe. In particular, it exists in halos that surround most galaxies. The mass of the halo exerts a strong gravitational pull on the galaxy itself and others in the vicinity. That’s pretty much the standard view of dark matter and its impact on galaxies. However, there are problems with the idea of ​​these halos. Apparently there are some oddly shaped dwarf galaxies that look like they don’t have halos. How could that be? Do they present an observational challenge to the prevailing notions of dark matter halos?

Find disturbed dwarf galaxies

In the so-called “Standard Model” of cosmology, shells or halos of dark matter protect galaxies from the gravitational influence of nearby galactic neighbors. However, when astronomers from the Universities of Bonn and Saint Andrews in Scotland looked into the nearby Fornax Cluster, which is about 62 million light-years from us, they saw something strange. It contains a number of dwarf galaxies with distorted, disordered shapes. This is strange, especially if they should be surrounded by dark matter halos.

Let’s take a quick look at dwarf galaxies. Small and faint, they are usually found in galaxy clusters or near much larger companions. The Milky Way is surrounded by a clique of dwarf galaxies. It actually cannibalizes the likes of the Sagittarius Dwarf Spheroid. Interestingly, recent studies show that at least one of the nearby dwarf galaxies, an old one called Tucana II, has an astonishingly massive dark matter halo.

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So what is happening in Fornax that is different? There, dwarf galaxies could be “disturbed” by gravitational tides from nearby larger ones in the cluster. Tides are formed when the gravity of one body acts differently on different parts of another body. These are similar to the tides on Earth when the moon pulls harder on the side of the earth facing it.

The distorted shapes of the dwarf galaxies observed by the team indicate a problem with our understanding of dark matter. “Such disturbances in the fornax dwarfs are not to be expected according to the standard model,” says Pavel Kroupa, professor at the University of Bonn and the Charles University in Prague. “Because according to this model, the dark matter halos of these dwarfs should partially protect them from the tides stirred up by the cluster.”

Explanation of distorted dwarf galaxies

Kroupa and Ph.D. Student Elena Ascencio analyzed observations of the disturbed dwarfs in Fornax. They wanted to understand how large the gravitational distortions of these galaxies are and what causes them. The levels of distortion that can be expected depend on a number of factors. One is the inner features of the dwarf galaxy. In addition, their distance from the center of the cluster is important. The gravitational influences are much stronger there. Typically, galaxies with large sizes but not many stars could easily be disturbed by strong gravitational tides. The same is true for galaxies closer to the core of the cluster.

Team members compared what they saw in the cluster to observations from the VLT Survey Telescope at the European Southern Observatory. Asencio pointed out that what they found seems to indicate problems with the Standard Model. “The comparison showed that if you want to explain the Standard Model observations,” she said, “the fornax dwarfs should already be destroyed by gravity from the cluster center, even if the tides throwing them at a dwarf are sixty-fourfold.” are weaker than the dwarf’s own gravity.”

Not only is this counterintuitive, she said, it also contradicts previous studies. The team also found that the force needed to perturb a dwarf galaxy is roughly equal to its own gravity.

What does this mean for the standard model?

The research team points out that it is difficult to explain these disturbed, disturbed shapes of the dwarf galaxies in Fornax when they are surrounded by dark matter. In other words, they shouldn’t be deformed if they have halos. Yet there they are with disturbed-looking shapes. This means that there are no dark matter halos around these galaxies.

Obviously, if what the astronomers found is confirmed, the Standard Model needs some adjustment. And there is at least one alternative explanation for the strange galaxy shapes. It’s called the MOND model (short for Modified Newtonian Dynamics). It suggests that Newton’s law of universal gravitation should be modified to take into account the observed properties of galaxies. It could be applied to explain why misshapen galaxies look the way they do.

According to Hongsheng Zhao, a member of the University of Saint Andrews research team, finding perturbed dwarfs without dark matter halos is a major challenge to the current view. It states that galaxies have halos. It seems that not everyone does, he points out. “Our results have major implications for fundamental physics,” he said. “We expect to find more disturbed dwarfs in other clusters, a prediction for other teams to check.”

For more informations

No sign of dark matter halos
The distribution and morphologies of the Fornax cluster’s dwarf galaxies suggest that they lack dark matter
Modified Newtonian dynamics