Nube, the almost invisible galaxy that defies the dark matter model

Nube is a nearly invisible dwarf galaxy discovered by an international research team led by the Instituto de Astrofísica de Canarias (IAC) in collaboration with the University of La Laguna (ULL) and other institutions.

The name was suggested by the 5-year-old daughter of one of the group’s researchers and is due to the object’s diffuse appearance. Its surface brightness is so weak that it had gone unnoticed during various previous studies of this part of the sky due to the diffuse appearance of the object, as if it were some kind of ghost. Indeed, its stars are so dispersed in such a large volume that “Nube” (Spanish for “Cloud”) was almost undetectable.

This newly discovered galaxy has a set of specific properties that distinguish it from previously known objects. The research team estimates that Nube is a dwarf galaxy 10 times fainter than other objects of its type, but also 10 times more extensive than other objects with a comparable number of stars.

To show what this means for anyone who knows a little about astronomy, this galaxy is a third the size of the Milky Way but has a mass similar to that of the Small Magellanic Cloud.

“With our current knowledge, we do not understand how a galaxy with such extreme characteristics can exist,” explains Mireia Montes, first author of the article and researcher at the IAC and ULL.

For several years, Ignacio Trujillo, the second author of the article, has been analyzing, using images from the Sloan Digital Sky Survey (SDSS), a specific strip of sky as part of the Legado del IAC Stripe 82 project. After reviews of the data, they noticed a slight blemish that seemed interesting enough to set up a research project.

The next step was to use ultra-deep multi-color images from the Gran Telescopio Canarias (GTC) to confirm that this area of ​​the survey was not an error type but an extremely diffuse object. Due to its weakness, it is difficult to determine the Nube’s exact distance.

From an observation obtained with the Green Bank Telescope (GBT) in the United States, the authors estimated Nube’s distance at 300 million light years, although future observations with the Very Large Array radio telescope ( VLA) and the William Herschel Optical Telescope (WHT) at the Roque de los Muchachos Observatory, La Palma, should help them demonstrate whether this distance is correct.

“If the galaxy turns out to be closer, it will remain a very strange object and will offer major challenges to astrophysics,” comments Ignacio Trujillo.

Another challenge for the current dark matter model?

The general rule is that galaxies have a much greater density of stars in their inner regions and that this density decreases rapidly as one moves away from the center. However, Montes says that in Nube, “the density of stars varies very little throughout the object, which is why it is so weak, and we were not able to observe it well until we had the ultra-deep images of the GTC.

Nube leaves astronomers perplexed. At first glance, the team explains, there is no interaction or other indication of its strange properties. Cosmological simulations cannot reproduce its “extreme” characteristics, even based on different scenarios. “We are left without a viable explanation within the currently accepted cosmological model of cold dark matter,” says Montes.

The cold dark matter model can reproduce the large-scale structures of the universe, but there are small-scale scenarios, like the Nube case, for which it cannot give a good answer. We have shown how different theoretical models cannot produce it, making it one of the most extreme cases known so far.

“It is possible that with this galaxy, and others like it that we might find, we could find additional clues that will open a new window on understanding the universe,” comments Montes.

“An interesting possibility would be that the unusual properties of Nube show us that the particles that make up dark matter have extremely small mass,” explains Ignacio Trujillo. If this were the case, the unusual properties of this galaxy would demonstrate the properties of quantum physics, but on a galactic scale. “If this hypothesis is confirmed, it would be one of the most beautiful manifestations of nature, unifying the world of the smallest with that of the largest,” he concludes.

The research is published in the journal Astronomy and astrophysics.