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

An multinational research team directed by the Instituto de Astrofísica de Canarias (IAC) in partnership with the University of La Laguna (ULL) and other institutions identified Nube, a virtually undetectable dwarf galaxy.

The term comes from the object's diffuse look and was proposed by the 5-year-old daughter of one of the group's researchers. The object seems hazy, almost like a ghost, and its surface brightness is so low that it was missed in the numerous prior scans of this region of the sky. This is because "Nube" (Spanish for "Cloud") was nearly invisible due to its stars' extreme dispersion across such a vast volume.

A number of unique characteristics make this recently found galaxy apart from other known objects. According to the study team, Nube is a dwarf galaxy that is both 10 times more extensive and 10 times fainter than other objects of its sort that have a similar amount of stars.

For those with a basic understanding of astronomy, this galaxy is equivalent in size to one-third of the Milky Way and has a mass comparable to the Small Magellanic Cloud.

"We do not know how a galaxy with such extreme characteristics can exist with our current knowledge," says Mireia Montes, the article's primary author and a researcher at the ULL and IAC.

As part of the Legado del IAC Stripe 82 project, Ignacio Trujillo, the article's second author, has been investigating a particular strip of sky using photos from the Sloan Digital Sky Survey (SDSS) for a number of years. They discovered a faint region in one of the data revisions that seemed intriguing enough to launch a study.

The next step was to verify that this patch in the survey was an exceedingly diffuse item rather than some kind of mistake using ultra-deep multicolor photos from the Gran Telescopio Canarias (GTC). The Nube is faint, making it difficult to measure its precise distance.

The authors estimated Nube's distance from Earth to be 300 million light years using an observation made with the Green Bank Telescope (GBT) in the United States. However, future observations with the Very Large Array (VLA) radiotelescope and the optical William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory, La Palma, should help them demonstrate whether this distance is accurate.

"If the galaxy turns out to be nearer, it will still be a very strange object and offer major challenges to astrophysics," says Ignacio Trujillo.

An further obstacle to the current dark matter model?

As a general rule, the number of stars in galaxies decreases quickly as one moves out from the center, with a considerably higher density seen in the inner regions. However, Montes states that in Nube, "the density of stars varies very little throughout the object, which is why it is so faint, and we have not been able to observe it well until we had the ultra-deep images from the GTC."

The astronomers are baffled by Nube. According to the scientists, there doesn't seem to be any interaction or other sign of its peculiar qualities. Its "extreme" features cannot be replicated by cosmological models, even when using diverse circumstances. "We are left without a viable explanation within the currently accepted cosmological model, that of cold dark matter," Montes says.

The cold dark matter hypothesis is able to replicate the universe's large-scale structures, but it is unable to provide a satisfactory explanation for certain small-scale events, as the Nube example. It is one of the most severe occurrences that has been observed to far, as we have demonstrated that it cannot be produced by any of the theoretical models.

"It is possible that with this galaxy, and similar ones which we might find, we can find additional clues which will open a new window on the understanding of the universe," Montes said.

"One possibility, which is attractive, is that the unusual properties of Nube are showing us that the particles that make up dark matter have an extremely small mass," claims Ignacio Trujillo. If this is the case, then the peculiarities of this galaxy would serve as a galactic example of quantum physics. "If this hypothesis is confirmed, it would be one of the most beautiful demonstrations of nature, unifying the world of the smallest with that of the largest," he says.

The research is published in the journal Astronomy & Astrophysics.