Unique Galaxy Has Less Dark Matter than Expected

Astronomers have determined that the galaxy NGC 1052-DF2, or DF2 for short, has 400 times less dark matter than expected for an object of its size.

In addition to providing insight into how galaxies form, the unusual galaxy is helping strengthen the argument for the existence of dark matter, researchers said.

According to the new paper's lead author, Pieter van Dokkum, a researcher from Yale University, the galactic find challenges the standard idea of how galaxies are born. While interactions between normal and dark matter have long been considered a key element in galaxy formation, the dearth of dark matter in this galaxy challenges that assumption.

"Dark matter is apparently not a requirement for forming a galaxy," van Dokkum told Space.com.

Dark matter is a form of matter whose presence is discernable only through its gravitational interactions with baryonic, or "normal," matter. The unseen matter seems to make up roughly 80 percent of the mass of the universe and is thought to play a key role in galactic evolution.

The mysterious material is considered the scaffolding of the universe. Small bits of dark matter clump together to create the bones of the scaffold, growing larger over time. This growth is accompanied by the formation of stars from the gas and dust of the galaxy. According to NASA, the interaction of stars and galaxies within the dark matter is thought to have produced the galaxies astronomers observe today.

But DF2 defies this idea. Van Dokkum and his colleagues were first drawn to the strange galaxy by a collection of 10 unusually bright compact objects orbiting around it. Using a combination of the light of the galaxy's stars and their color, the scientists measured how much normal mass could be found within the galaxy. They found that DF2 is about the size of the Milky Way but has about 200 times fewer stars.

The mass of the combined stars is about 200 million times the mass of Earth's sun, a unit known as a solar mass. The researchers then used the motion of the bright objects, classified as globular clusters, to calculate the total mass in the galaxy.

"For a galaxy with a stellar mass of about 200 million solar masses, we expect a dark matter mass of about 80,000 million solar masses," van Dokkum said. The total mass of the system, however, weighed in at no more than 300 million solar masses, significantly less than anticipated.

If dark matter is a key ingredient for galaxy formation, then how did DF2 form? The researchers suggested that DF2 could be an old tidal dwarf galaxy, formed from the gas cast out of other merging galaxies. DF2 lies not far from another elliptical galaxy that could have donated the material in a previous merge, the researchers said.

Another potential explanation is that winds blowing through the interstellar medium swept up enough gas to build the unusual galaxy, an explanation also strengthened by DF2's close neighbor. Material flowing in toward the neighboring galaxy could have fragmented as well, helping to form the unique object.

"We thought all galaxies were made up of stars, gas and dark matter mixed together, but with dark matter always dominating," Roberto Abraham, co-author on the study and a researcher at the University of Toronto, said in a statement. "Now, it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter. It is pretty bizarre."

The research was published online on March 28 in the journal Nature.