Astronomers Find a Galaxy Without Dark Matter for the First Time

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Scientists have various theories on how this galaxy came to be.

Dark matter, which is invisible, is thought to comprise about a quarter of the universe's combined mass and energy and about 80% of its total mass, but has not been directly observed. Finding such a thing would be like finding smoke but no fire, effect without cause.

Van Dokkum said NGC1052-DF2 is so sparse that "it is literally a see-through galaxy". In addition to the dearth of dark matter, DF2 is unusual in another way: it is roughly the size of our Milky Way Galaxy, but contains only 1/200 the number of stars. 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. In an odd way, the new galaxy's lack of dark matter may serve as evidence for the existence of it by demonstrating that there is much astronomers do not understand about such vast low-density galaxies. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. In 2016, they found the opposite of this galaxy-one that was rotating so fast that they concluded it was 99.99 percent dark matter.

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

Van Dokkum and his colleagues were not looking for a dark-matter-deficient oddball when they started studying NGC 1052-DF2. Surely, astronomers thought, dark matter must play a role in forming these objects so devoid of normal star stuff.

The ultra-diffuse galaxy at left is rich in globular clusters, which hold the key to understanding this mysterious object's origin and mass. Small bits of dark matter clump together to create the bones of the scaffold, growing larger over time.

NGC 1052-DF2 is missing its dark matter.

Parsing what that means isn't easy, but it does lead to one clear, if counterintuitive, conclusion, the team argues: "Alternatives to dark matter have trouble with this object", van Dokkum said. NGC 1052-DF2 resides about 65 million light-years away in a collection of galaxies that is dominated by the giant elliptical galaxy NGC 1052. Three of them appear similar to NGC 1052-DF2. To follow up on their observations the researchers used the W.M. Keck Observatory in Hawaii to measure the movement of dense groups of stars called globular clusters.

The point sources proved to be 10 globular clusters-compact and spherical groupings of stars orbiting the galaxy's core. By measuring their motions, the astronomers could calculate the mass of material enclosed inside their orbits. They found that the clusters were moving at relatively low speeds - less than 23,000 miles per hour.

These ideas, however, still do not explain how this galaxy formed.

Kathryn Zurek, an astrophysicist at the Lawrence Berkeley National Laboratory who did not work on the paper, wrote in an email to Live Science that the discovery "looks on the surface to be another nail in the coffin of MOND", because "the absence of DM [dark matter] in this galaxy shows that baryons [normal matter] and DM indeed behave as separate substances". It could be that NGC 1052-DF2 was once a placid mass of gas and has been recently perturbed by another unseen galaxy nearby, sparking star formation. "Now it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter".

The obvious question is how you end up with a galaxy like NGC1052-DF2.

One outsider suggested that perhaps the "galaxy" van Dokkum studied is so diffuse that it may not really be a galaxy. 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.