Ghost-like Galaxy Defies Dark Matter Model

Astronomers have discovered a ghost-like galaxy about half as large as our own Milky Way but no more massive than the puny Small Magellanic Cloud, our galaxy’s dwarf satellite. Since the new galaxy’s stars are spread out over a huge volume, it’s invisible to most telescopes, like a Halloween specter. The origin of Nube (Spanish for “cloud”), as astronomers are calling the new find, may challenge popular ideas about the nature of dark matter.

Over the past years, facilities like the Dragonfly Telephoto Array in New Mexico (see May 2019 issue of Sky & Telescope) first revealed the existence of ultra-diffuse galaxies (UDGs) that exhibit an unusually low surface brightness. However, Nube is even more extreme. “This is a particularly large and faint example,” says Dragonfly initiator Pieter van Dokkum (Yale University), who was not involved in the new discovery. “I was excited when I saw it presented at a conference on ultra-diffuse galaxies this summer in Sesto, Italy. We keep finding larger and fainter systems, and we don’t really know where the limits are!”

An international team led by Mireia Montes (Institute of Astrophysics of the Canaries, Spain) serendipitously found the faint blob in a deep survey of an equatorial strip of sky within the constellation Cetus, the Whale. Follow-up radio observations with the 100-meter Green Bank Telescope in West Virginia yielded the galaxy’s redshift, which corresponds to a distance of some 350 million light-years. Using deep, multi-color imaging with the 10.4-meter Gran Telescopio Canarias at La Palma, Montes and her colleagues estimate a stellar mass of some 400 million solar masses and an age of 10 billion years. The results will appear in Astronomy & Astrophysics (preprint available here).

“This group is one of the foremost teams in the world studying low-surface-brightness phenomena,” comments van Dokkum, “and I am happy to see that they are also finding interesting ultra-diffuse galaxies. I think they were somewhat skeptical about [our earlier work] in the past, and that actually greatly helped push the field forward, as it led to a lot of follow-up work that otherwise might not have happened.”

Despite the new finds, however, the origin of ultra-diffuse (or almost dark) galaxies is still confounding astronomers. In the case of Nube (a name proposed by the five-year-old daughter of team member Ignacio Trujillo), Montes and her colleagues present convincing evidence that it was born as an isolated object. In other words, it’s not a tidal dwarf galaxy, forming in the aftermath of an encounter between larger galaxies. Moreover, its extremely regular, symmetrical shape suggests it never experienced interactions that might explain its weird properties. “I don’t have a good explanation for the origin,” says Montes.

“It sometimes seems that every UDG has its own story to tell,” observes Van Dokkum. “Some are puffed up or ripped apart by tidal forces, some have no dark matter, others appear to be ‘failed galaxies,’ with many globular clusters and an overabundance of dark matter. It is not yet clear in what category Nube falls, I think.”

One thing is clear, though: Nube contains loads of dark matter, most likely distributed in a large halo. The Green Bank observations reveal the dynamical properties of the system’s neutral hydrogen gas, and they indicate a total galaxy mass of more than 25 billion solar masses – about 25 times more than the stars and hydrogen gas combined.

However, there’s a problem: Cosmological simulations based on current dark matter theories (in which the mysterious stuff consists of weakly interacting massive particles, or WIMPs) fail to produce galaxies like the new find. Simulated galaxies with stellar masses and dark matter halo masses like Nube invariably turn out to be much smaller.

“We tried to look at all the scenarios that could create something extended,” says Montes, “but nothing could really explain Nube. So we decided to see if other forms of dark matter could do it.”

Indeed, if dark matter consists of extremely low-mass, axion-like particles (also known as fuzzy dark matter), the observed properties of Nube can be reproduced. “However, the situation is far from clear,” Montes’ team admits. “Although fuzzy dark matter could relieve some of the [problems] appearing in the cold dark matter scenario, more work is needed to assess this model.”

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Editorial note (Oct. 31, 2023): This story has been updated with quotes from team lead Mireia Montes.