Scientists stunned by discovery of galaxy without dark matter

"NGC1052-DF2 challenges the standard ideas of how we think galaxies form," said astronomer Pieter van Dokkum.

By Brooks Hays
All of the ultra-diffuse galaxy's mass can be accounted for by its stars. Photo by Gemini Observatory/NSF/AURA/Keck/Jen Miller
All of the ultra-diffuse galaxy's mass can be accounted for by its stars. Photo by Gemini Observatory/NSF/AURA/Keck/Jen Miller

March 28 (UPI) -- Most galaxies are defined by their dark matter. To study dark matter -- and affirm its existence -- astronomers study galaxies.

Now, for the first time, scientists have found a galaxy without dark matter -- NGC1052-DF2's dark matter is missing. When astronomers first found the dark matter-less galaxy, they were stunned.


"For decades, we thought that galaxies start their lives as blobs of dark matter," Pieter van Dokkum of Yale University said in a news release. "After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC1052-DF2 challenges the standard ideas of how we think galaxies form."

The galaxy is an ultra-diffuse galaxy, it's large and faint, and is located 65 million light-years away. Though ultra-diffuse galaxies were only recently discovered, they are relatively common. But NGC1052-DF2 is the first to be found without dark matter.

"NGC1052-DF2 is an oddity, even among this unusual class of galaxy," said Yale grad student Shany Danieli.

Astronomers were first alerted to the galaxy's strange composition when they noticed discrepancies in the observations made by the Dragonfly Telephoto Array and the Sloan Digital Sky Survey. The Dragonfly images revealed a faint, blob-like object, while SDSS renderings showed a collection of bright point-like sources.


To further explore the discrepancy and study the unusual internal structure of NGC1052-DF2, astronomers observed the galaxy using the Gemini Multi Object Spectrograph, Keck's Deep Imaging Multi-Object Spectrograph and Low-Resolution Imaging Spectrometer.

"Without the Gemini images dissecting the galaxy's morphology we would have lacked context for the rest of the data," said Danieli. "Also, Gemini's confirmation that NGC1052-DF2 is not currently interacting with another galaxy will help us answer questions about the conditions surrounding its birth."

The Keck data showed the point-like sources, the globules, were moving much slower than astronomers expected. The movement and speed of a galaxy's components allow scientists to measure the galaxy's mass. Astronomers determined the galaxy's stars accounted for all of NGC1052-DF2's estimated mass. There was no dark matter to be found.

Researchers described their discovery in the journal Nature.

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

No one is really sure how the galaxy formed without dark matter.

The presence of the nearby giant elliptical galaxy NGC 1052, home to violent galactic formation and evolution, could explain NGC1052-DF2's lack of dark matter. It's also possible a sudden burst of stellar formation swept away the galaxy's gas and dark matter, stunting its development.


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