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Astronomers are astounded by distant galaxies without dark matter



WASHINGTON – It's a double cosmic puzzle: many things that were already invisible have disappeared.

Astronomers have found a distant galaxy in which there is no dark matter.

Dark matter is called "dark" because it can not be seen. It is the mysterious and invisible skeleton of the universe that scientists say is about 27 percent of the cosmos. Scientists only know that dark matter exists because they can observe how they push and pull things that they can see, like stars.

It should be everywhere.

But the astronomer at Yale University, Pieter van Dokkum and his colleagues, spied on a huge, old galaxy with relatively few stars, showing what you really see. According to a study published Wednesday in the journal Nature, the stars of the galaxy race without any visible influence from the dark matter.

Rather than shaking the foundations of physics, scientists say that this absence of dark matter could help prove the existence of dark matter.

"I'm not sure what to make of it, but it's definitely fascinating," wrote Case Western Reserve astronomer Stacy McGaugh, who was not part of the study, in an email. "This is a weird galaxy."

Van Dokkum investigates diffuse galaxies that cover huge areas but have relatively few stars. In order to search for them, he and his colleagues built a self-made telescope out of 48 telephoto lenses, which he first illuminated with a toy lamp on a paperclip. The Beetle-Eyed Telescope, called Dragonfly, looks up from New Mexico to the sky.

With Dragonfly, van Dokkum and his colleagues found a large, sparse galaxy called NGC1

052-DF2 in the northern constellation Cetus, also known as the whale. It's the size of the Milky Way, but with only one percent of its stars. Then they used larger telescopes in Hawaii and finally the Hubble Space Telescope to study the galaxy.

Although the galaxy is largely empty, they found clusters of densely clustered stars. Using measurements on the telescopes, van Dokkum and colleagues calculated how fast these clusters moved. If there were a normal amount of dark matter, those piles would bounce at around 67,000 miles an hour. Instead, the clusters moved at around 18,000 mph. That's about as fast as they would move if there were no dark matter at all, van Dokkum said.

The team also calculated the total mass of the galaxy and found that the stars are responsible for everything, with little or no room for dark matter.

"I find that unlikely in all possible contexts," said McGaugh, who is a proponent of a "modified gravity" theory that completely rules out the existence of dark matter. "That does not make it wrong, just really weird."

How could this absence of Dark Matter prove it exists? By refuting modified theories of gravitation, which suggest that gravitation works in such a way that the cosmos makes sense without dark matter. But these alternative theories require that stars in this galaxy break at least twice as fast as observed in this study.

Other external scientists said the first look at the calculations seems correct, although the results are confusing. A galaxy with so few stars should have more dark matter than others, none.

"These are very strong scientists and that's why I take the results very seriously," said Marc Kamionkowski, a physicist at Johns Hopkins University.

An outsider thought that the "galaxy" of Dokkum might be so diffuse that it is not really a galaxy. Another suggested that the dark matter could only be outside the range van Dokkum measured.

Van Dokkum rejected both options. "It's not negotiable, there's nothing else, just the stars," he said. The only way to explain this, if dark matter exists in the universe, is not in this galaxy, he said.

There is no good explanation for why and how this galaxy has no dark matter, van Dokkum said. He suggested four different options – all unproven. His favorite: That the galaxy was formed in the early universe in a way that astronomers have never seen or understood.

"It's not that often that you get a real surprise," van Dokkum said.


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