Galaxy has something missing: Dark matter

Astronomers struggling to explain ‘weird’ setup

WASHINGTON — It’s a double cosmic conundrum: Lots of stuff that was already invisible has gone missing.

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

Dark matter is called “dark” because it can’t be seen. It is the mysterious and invisible skeleton of the universe that scientists figure makes up about 27 percent of the cosmos. Scientists only know dark matter exists because they can observe how it pushes and pulls things they can see, like stars.

It’s supposed to be everywhere. But Yale University astronomer Pieter van Dokkum and colleagues spied a vast, old galaxy with relatively few stars where what you see truly is what you get. The galaxy’s stars are speeding around with no apparent influence from dark matter, according to a study published in Wednesday’s journal Nature.

“Not sure what to make of it, but it is definitely intriguing,” wrote Case Western Reserve astronomer Stacy McGaugh, who was not part of the study, in an email. “This is a weird galaxy.”

Van Dokkum studies diffuse galaxies, ones that cover enormous areas but have relatively few stars. To look for them he and colleagues built their own makeshift telescope out of 48 telephoto lenses that he tested by using a toy flashlight to shine a light on a paper clip. The bug-eyed telescope, called Dragonfly, peers into the sky from New Mexico.

Using Dragonfly, van Dokkum and colleagues found a large, sparse galaxy called NGC1052-DF2 in the northern constellation Cetus. It’s as big as the Milky Way but with only 1 percent of its stars. Then they used larger telescopes on Hawaii and the Hubble Space Telescope to study the galaxy.

Though the galaxy is mostly empty, they found clusters of densely grouped stars. With measurements from the telescopes, van Dokkum and colleagues calculated how fast those clusters moved. If there were a normal amount of dark matter those clusters would be speeding around at about 67,000 mph. Instead, the clusters were moving at about 18,000 mph. That’s about how fast 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 the stars account for everything, with little or no room left for dark matter.

“I find this unlikely in all possible contexts,” said McGaugh, who is a proponent of a “modified gravity” theory that excludes the existence of dark matter altogether. “That doesn’t make it wrong, just really weird.”

There’s no good explanation for why and how this galaxy has no dark matter, van Dokkum said. He proposed four different possibilities — all unproven. His favorite: That the galaxy formed in the very early universe in a way astronomers have never seen or understood.

“It’s not so often you get a true surprise,” van Dokkum said.