Published: Sun, April 01, 2018
Research | By Dana Schwartz

A Ghostly Galaxy Lacking Dark Matter (image)

A Ghostly Galaxy Lacking Dark Matter (image)

At the beginning of everything, those simulations suggest, all the matter we see in the galaxy was spread out in a thin gas. Normally, we infer that there's dark matter around because the galaxy appears to have a lot more matter than the amount provided by the stars we can see.

"In other words, no dark matter, no galaxy", says National Geographic.

When Prof Pieter van Dokkum, lead author of the study, first spotted NGC1052-DF2: "I stared a lot at that image and just marvelled at it..." But observations through telescopes show them spinning way faster than we'd expect, as if they were actually much more massive than the stars we can see indicate.

Despite how little we know about dark matter, we can tell when it's there, from the way it impacts galaxies and causes unusual movements that don't fit the laws of gravity. This invisible, mysterious substance is the most dominant aspect of any galaxy.

The thing about dark matter, or whatever's causing outer space to move so weirdly, is that it's predictably distributed throughout the universe.

The unique galaxy, called NGC 1052-DF2, contains at most 1/400th the amount of dark matter that astronomers had expected.

The team is already hunting for more dark-matter deficient galaxies. Surely, astronomers thought, dark matter must play a role in forming these objects so devoid of normal star stuff.

Ultra-diffuse galaxies are oddities in their own right, having only been discovered in 2015 as they are very hard to detect. When van Dokkum and his team found NGC 1052-DF2, they expected to see something similar. "It's not something we were looking for or expecting". Hubble helped to accurately confirm the distance of NGC 1052-DF2 to be 65 million light-years and determined its size and brightness. Dragonfly saw it as a diffuse object with some structures in it; Sloan imaged it as a collection of distinct objects.

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The point sources proved to be 10 globular clusters-compact and spherical groupings of stars orbiting the galaxy's core. Keck revealed that the globular clusters were moving at relatively low speeds, less than 23,000 miles per hour.

The team is now proposing additional observations of NGC 1052-DF2, including with the upcoming, albeit oft-delayed, James Webb Space Telescope. For example, it could be that gas was pulled out of a big galaxy in a merger with another galaxy.

"It is conventionally believed to be an integral part of all galaxies, the glue that holds them together and the underlying scaffolding on which they are built", said co-author Allison Merritt from the Max Planck Institute for Astronomy, in Germany. By following the movement of 10 inserted star clusters, the group could decide how much mass is tucked into the system.

Astronomers first discovered the galaxy in 2000 as a smudge on old photographic plates. If it has no dark matter, how did it even evolve into a galaxy? The astronomers speculate that the birth and formation of DF2 in the dynamic environment of the cluster could have been influenced by the giant galaxy. "The standard paradigm has gas falling into halos - or lumps - of dark matter and turning into stars to make a galaxy", he said.

The obvious question is how you end up with a galaxy like NGC1052-DF2. Because there's some regular issue right here, any kind of variation of customized gravity would certainly have that issue generate dark-matter-like impacts. Now researchers are pondering possible explanations for this missing dark matter in NGC 1052-DF2. Dark energy, according to this understanding, is about 71 percent of everything. Yet in this odd galaxy, the projected signatures of these exotic effects are not seen. Despite it being so elusive, its effects have been identified through several different methods, and there is very strong evidence to suggest that dark matter does exist. "It can be in a galaxy or not in a galaxy, but it is not a field, or some alternative thing that manifests itself rather than being a substance".

"This has been a common experience of mine: When I find something that doesn't make sense in MOND, it usually doesn't make sense in dark matter, either", he wrote. "There might be a change in the ways that gravity works, compared to what Einstein and Newton would have said, but let's not immediately draw conclusions", he says.

The Hubble Space Telescope is a project of global cooperation between NASA and ESA (European Space Agency). In fact, every galaxy that astronomers have ever studied contains dark matter.

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