Dark Matter Goes Missing in Oddball Galaxy

Researchers were surprised when they uncovered a galaxy that is missing most, if not all, of its dark matter. An invisible substance, dark matter is the underlying scaffolding upon which galaxies are built. It's the glue that holds the visible matter in galaxies — stars and gas — together.

This large, fuzzy-looking galaxy is so diffuse that astronomers call it a “see-through” galaxy because they can clearly see distant galaxies behind it. Image credits: NASA, ESA, and P. van Dokkum (Yale University)

The unique galaxy, called NGC 1052-DF2, contains at most 1/400th the amount of dark matter that astronomers had expected. The galaxy is as large as our Milky Way, but it had escaped attention because it contains only 1/200th the number of stars. Given the object's large size and faint appearance, astronomers classify NGC 1052-DF2 as an ultra-diffuse galaxy.

None of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball.

The team used the W.M. Keck Observatory in Hawaii to measure the motions of 10 giant groupings of stars called globular clusters in the galaxy. The globular clusters were moving at relatively low speeds, less than 23,000 miles per hour.

Stars and clusters in the outskirts of galaxies containing dark matter move at least three times faster. They calculated the galaxy's mass. The stars in the galaxy can account for all the mass, and there doesn't seem to be any room for dark matter.

The galaxy it is a gigantic blob that you can look through. It's so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.

The ghostly galaxy doesn't have a noticeable central region, or even spiral arms and a disk, typical features of a spiral galaxy. But it doesn't look like an elliptical galaxy, either. The galaxy also shows no evidence that it houses a central black hole. Based on the colors of its globular clusters, the galaxy is about 10 billion years old. Even the globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies.

NGC 1052-DF2 resides about 65 million light-years away in a collection of galaxies that is dominated by the giant elliptical galaxy NGC 1052. Galaxy formation is turbulent and violent, and the growth of the fledgling massive galaxy billions of years ago perhaps played a role in NGC 1052-DF2's dark-matter deficiency.

Another idea is that gas moving toward the giant elliptical NGC 1052 may have fragmented and formed NGC 1052-DF2. The formation of NGC 1052-DF2 may have been helped by powerful winds emanating from the young black hole that was growing in the center of NGC 1052. These possibilities don't explain all of the characteristics of the observed galaxy. (NASA)

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