Bizarre 'ghost galaxy' leaves scientists in the dark


A galaxy far far away has baffled astronomers because it contains almost no dark matter, making it one of the strangest objects in the universe.

Normally the mysterious substance, whose nature is unknown but which exerts a strong gravitational influence, dominates galaxies including our own Milky Way.

Until now it was thought to be essential to galaxy formation. Blobs of dark matter were understood to act as galaxy "seeds", gathering in the normal matter from which stars are made with their gravity.

Composite colour image of 'ghost galaxy' DF2. The galaxy is unique in that it contains almost no dark matter. (Gemini Observatory/NSF/AURA/Keck/Jen Miller/PA)
Composite colour image of 'ghost galaxy' DF2. The galaxy is unique in that it contains almost no dark matter. (Gemini Observatory/NSF/AURA/Keck/Jen Miller/PA)

With virtually no dark matter, scientists are at a loss to explain how the newly identified "ghost" galaxy was created.

Lead scientist Professor Pieter van Dokkum, from Yale University in the US, said: "There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown."

NGC1052-DF2, or "DF2" for short, is 65 million light years away and one of a newly recognised family of "ultra-diffuse" galaxies.

They are thought of as "ghostly" because they contain so few stars, making them very faint despite their large size.

DF2 is about the same size as the Milky Way but contains 200 times fewer stars, as well as almost no dark matter.

The weird object was first spotted by astronomers using the Dragonfly Telephoto Array, a special 48-lens telescope in New Mexico, US, designed to find ultra-diffuse galaxies.

Follow-up observations revealed a number of unusual characteristics. Unlike typical spiral galaxies, DF2 had no dense central region, and lacked spiral arms or a disc. And unlike elliptical galaxies it had no central black hole.

But the oddest thing about DF2 was revealed when astronomers estimated the galaxy's mass by measuring the speed at which clusters of stars were orbiting its centre. Slower velocity equated to greater mass.

They found that almost all of DF2's mass could be accounted for by visible stars, gas and dust. The expected "invisible" mass consisting of dark matter was not there.

"If there is any dark matter at all, it's very little," said Prof van Dokkum. "The stars in the galaxy can account for all of the mass, and there doesn't seem to be any room for dark matter."

Describing DF2 as "astonishing", he said the galaxy was so sparse it was possible to see right through it.

He spent an hour "just staring" at an image of the object from the Hubble Space Telescope which showed "a gigantic blob that you can look through". All the galaxies behind DF2 were visible.

The discovery is reported in the journal Nature. Scientists are now trying to come up with a viable explanation for the galaxy's lack of dark matter.

One clue may be that DF2 sits in a collection of galaxies dominated by a giant elliptical galaxy, NGC 1052. The turbulent birth of NGC 1052 billions of years ago may have played a role in DF2's dark matter deficiency, astronomers believe.

Another possibility is that some cataclysmic event within DF2 swept out all of its gas and dark matter, halting star formation.

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