Galaxy seems to lack dark matter, stumping astronomers

Galaxy seems to lack dark matter, stumping astronomers

Galaxy seems to lack dark matter, stumping astronomers

"NGC1052-DF2, however, challenges these standard ideas about how galaxies are formed", says Pieter van Dokkum. That is, except for one. A Yale-led research team made the discovery while peering at the distant galaxy NGC 1052-DF2.

NGC 1052-DF2 resides about 65 million light-years away in the NGC 1052 Group, which is dominated by a massive elliptical galaxy called NGC 1052.

The absence of this mysterious stuff in NGC 1052-DF2, described Wednesday in the journal Nature, could shed light on galaxy formation and help scientists narrow down what dark matter actually is.

Many scientists believe that dark matter - matter that scientists can not see directly - is everywhere in the universe, and helps explain how galaxies hold together. So finding a galaxy without it is unexpected. It is real, but it has its own distinct existence from the other components of the galaxies.

In that study, the researchers found an object that could exist in a universe that has dark matter, but that would be almost unimaginable in a MOND universe: a totally normal galaxy, one that seems to operate without any dark matter-type forces. By looking at the dynamical properties of ten globular clusters - essentially, a very large ball of stars - orbiting the galaxy, it allowed the team to work out an independent value of the galaxies mass. But upon further study, an worldwide team of astronomers was surprised to discover that it had no dark matter, something that has been seen as crucial in the formation of galaxies. "I spent an hour just staring at the Hubble image", van Dokkum recalled. "It's so rare, particularly these days after so many years of Hubble, that you get an image of something and you say, 'I've never seen that before'". It's a literally a transparent galaxy. "There's a pretty tight relationship between the amount of stars that formed and the dark matter there, at least when the galaxy formed". Weird and invisible, dark matter has not been directly observed-no one is sure what it is, exactly-but it is key to explaining the movement of galaxies in space.

So-called ordinary matter - including stars, gases, dust, planets and everything on them - accounts for only five percent of all content in the Universe.

Ordinarily, not the greater part of a galaxy's mass is obvious. "How do you form such a thing?"

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"It's not just galaxies", explains van Dokkum. The galaxy is a complete mystery, as everything about it is unusual.

The galaxy had been imaged before but on second look, the team noticed it looked very different from other imaged galaxies. If astrophysicists find more similar galaxies, they'll have to revise their current theories about dark matter.

The MOND theory - Modified Newtonian Dynamics - suggests that the phenomena usually attributed to dark matter can be explained by modifying the laws of gravity.

Based on its its size and brightness, NGC 1052-DF2 is classified as an ultra diffuse galaxy that is larger than the Milky Way, but contains about 250 times fewer stars.

Dr Richard Massey, a physicist at Durham University, agrees: "I'm genuinely very impressed with the work, and I'd use the conclusions to say that we should stare at these objects a lot harder for a lot longer - but I wouldn't conclude anything profound about dark matter quite yet", he told the BBC.

Scientists spoke of "a galaxy of absolute mystery, not predicted by any theory, everything on it is odd". But dark matter theorists have relied heavily on the idea of "abundance matching", which links the masses of dark matter halos closely to the masses of the galaxies they surround.