See-Through Galaxy Has Almost No Dark Matter — What’s Going On?

The galaxy NGC 1052-DF2 lies about 65 million light-years away in the constellation Cetus the Sea Monster. The galaxy is unusually transparent — you can see more distant background galaxies shining right through it. The galaxy is also missing most of its dark matter, the “glue” that holds most galaxies together in one piece. How it formed is a mystery. NASA / ESA / P. van Dokkum

An international team of researchers have uncovered an amazing new creature in the cosmic zoo — a galaxy relatively close to the Milky Way that’s missing most if not all of its dark matter. The discovery, which appeared on March 28 in the journal Nature, has quickly gotten astronomers’ attention. Current theory requires dark matter as a “seed” to initiate the formation of a galaxy. Although we can’t see dark matter, it does attract visible matter and is believed responsible for the formation of the galaxies when the universe was young.

Put simply, theory doesn’t predict rogue galaxies that coalesce without the help of invisible dark matter. If that’s not weird enough, you can practically see through NGC 1052-DF2. Located 65 million light years away, the team determined that the puffy thing is larger than the Milky Way but contains 250 times fewer stars.

“I spent an hour just staring at this image,” said lead researcher Pieter van Dokkum of Yale University. “This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy.”

Astronomers determined NGC 1052-DF2’s mass by measuring the motions of globular clusters (spherical clusters containing tens of thousands of stars) moving around the galaxy’s center. It turns out to be about equal to the mass of the visible stars in the galaxy. They were supposed to get a MUCH larger number had dark matter been there as predicted. Instead, they were forced to conclude that this puffball has at least 400 times less dark matter than astronomers predict for a galaxy of its mass, and possibly none at all.

“Dark matter is conventionally believed to be an integral part of all galaxies — the glue that holds them together and the underlying scaffolding upon which they are built,” said co-author Allison Merritt. He added: “This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work.”

NGC 1052-DF2 is found near the giant galaxy, NGC 1052. Did the bigger one steal dark matter from the little one? DSS2

So now scientists are faced with a wonderfully infuriating prospect — how to re-work the theory galaxy formation to account for something that shouldn’t be there. The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies.

Juices flowing, researchers already have some ideas to explain the missing dark matter such as a massive wave of star formation that swept out all the gas and dark matter, leaving a giant but dilute nothing in its wake. Or did the growth of the nearby massive elliptical galaxy NGC 1052 billions of years ago remove dark matter through some sort of gravitational interaction?

To find an explanation, the team is already hunting for more dark-matter deficient galaxies as they analyze 23 additional ultra-diffuse galaxies with the Three of them appear to be similar to NGC 1052-DF2. Aha!

PS. Thank you all for your kind remarks and support on Facebook and via e-mail regarding the publication of my new book. It means a lot to me.