Astronomers are puzzling over the newly discovered “Nobby” galaxy
Because it glows faintly in the sky and cannot be explained using theoretical models, the galaxy raises questions. Will the secret of dark matter be revealed?
Composition of the area around the Nube Galaxy: The background is black and white, and the galaxies in the background are shown in colour. Nobby in the centre.
Image: Montes et al., Astronomy and Astrophysics
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In a perfectly ordinary galaxy like the Milky Way, more than 100 billion stars shine. This is a number with eleven zeros – although only a few thousand of them can be seen with the naked eye on clear nights. One might think that the twinkling of stars is a feature of the galaxy if there were not also strange counterparts: dark galaxies.
The recently discovered Nube Galaxy glows at a fraction of the brightness of the Milky Way, according to one report in February. A study published in the specialized journal “Astronomy and Astrophysics”.. The Nube Galaxy simply lacks the usual number of stars for its size, according to the study's authors from the Institute of Astrophysics of the Canary Islands and the University of La Laguna.
Because of its unusual nature, the team led by study leader Mireya Montes tried to simulate the galaxy using computer models. However, only contradictory results have emerged – for astrophysicists, a sign that Nobby could hold a key to the fundamental nature of galaxies and lead to one of the greatest mysteries of the universe: dark matter.
A strange diffuse spot in the starry sky
How strange Nube is can also be seen in the way astronomers in the Canary Islands spotted it in the sky. Large-scale astronomical projects conducting systematic, computer-assisted sky surveys such as the Sloan Digital Sky Survey have so far been unable to detect this type of galaxy. Serendipity came to the aid of Montes' team. While analyzing astronomical data from a section of the starry sky, they noticed a diffuse spot that later turned out to be a galaxy. They named the spongy body “Nobi” after the young daughter of one of the study’s authors.
In order to examine the newly discovered galaxy more closely, astronomers reserved observation time in January 2019 for the largest optical telescope and the largest fully mobile radio telescope in the world: the Gran Telescopio Canarias with its mirror more than ten meters long on Canary Island. Tenerife and the Green Bank 100-meter radio telescope in the USA.
In this way, the researchers discovered something they did not expect. Nubia, which weighs 400 million solar masses, spans an area three times larger than dark galaxies of similar mass, but has only a tenth its usual luminosity. The galaxy also turned out to be semi-transparent, which means researchers were able to see objects located behind Nobby.
How could stars form in Nobi?
Despite all this, Nobby is a real galaxy. “‘Dark’ does not mean that there are almost no stars, but that they are distributed over such a large area that the galaxy as a whole shines only weakly,” says astrophysicist Andreas Burkert of Ludwig Maximilian University in Munich. “Compared to other typical galaxies, the conditions here are extreme. How stars can form there is a mystery to me.
Normally, stars form from vast, dense clouds of gas that collapse under their own gravity until they reach a mass and density that ignites a stellar fire in which hydrogen fuses together to form helium. “But if it's as big as Nobby's, gravity isn't enough to collapse the gas cloud and form a star,” Burkert says.
In fact, it wasn't the Nube stars that interested the study's authors, but rather the role played by dark matter. “Low-mass galaxies are ideal laboratories to better understand dark matter,” says Montes, the study leader. “Such galaxies, especially their centers, are dominated by dark matter 100 or 1,000 times more visible stellar mass.” According to Montes, the lower the density of stars, the easier it is to explain the effect of dark matter, because visible matter does not affect the shape and behavior of the galaxy.
To explain what it looked like, Montes' team fed data from Nube to their computers. In the first simulation, the team used so-called cold dark matter. In this theoretical model, physicists assume that dark matter particles must be very heavy in order to generate corresponding gravity. However, at the same time, it would have to be icy cold to remain invisible to human measuring instruments. If the atmosphere were warm, radiation would be emitted that could be measured.
A contradiction to the usual dark matter model
However, it turns out that Nube cannot be explained using this traditional model. The result was hypothetical galaxies that were similar in appearance but very small. According to the usual models, a galaxy's dark matter is concentrated at its center, while its density decreases rapidly outward. However, in the case of NOPE, they are relatively uniformly distributed in the nucleus, but this cannot be reconciled with the theoretical behavior of cold dark matter.
Therefore, Montes and her colleagues changed a fundamental property of dark matter, perhaps the most important quantity in such simulations. Instead of cold dark matter, they formed galaxies on the computer using “mysterious dark matter,” diffuse dark matter. In this theoretical model, dark matter particles have completely different properties and are widely dispersed in the cores of galaxies. In fact, the authors were able to create Nubian-like galaxies that were finally the right size. “We can only explain their extreme properties through models of dark matter other than classical ones,” Monti says, summarizing their results.
Is diffuse dark matter an aberration?
On the other hand, Burkert, himself a specialist in astrophysical simulations, considers diffuse dark matter to be a deviation to explain the Nubian shape. “The conclusion that it is mysterious dark matter is wrong,” Burkert says. Perhaps the mass of dark matter could be determined this way for one galaxy, but it would not be suitable for other galaxies. “If every galaxy has its own dark matter mass, it doesn't make sense.”
Montes also realizes that her simulations are flawed and that more work is needed to understand dark matter and thus dark galaxies. For Burkert, it represents a unique opportunity for science. “Our models are so well balanced for standard galaxies that they always fit them,” Burkert says. “But that doesn’t mean they perfectly represent reality.” “Only when our models fail for a galaxy do we realize that we have made too many assumptions and don’t really understand them. We learn a lot about physics from these alien species.
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