Astronomers using the NASA/ESA Hubble Space Telescope have discovered a unique galaxy — the first of its kind — that appears to contain virtually no dark matter. This discovery, reported in the journal Nature, challenges currently-accepted theories of galaxy formation and provides new insights into the nature of dark matter.
The NASA/ESA Hubble Space Telescope took this image of NGC 1052-DF2 on November 16, 2017. Image credit: NASA / ESA / P. van Dokkum, Yale University.
Named NGC 1052-DF2, the unique galaxy lies in the constellation of Cetus, about 65 million light-years away.
It is roughly the size of our Milky Way Galaxy, but contains only 1/200 the number of stars.
Given the object’s large size and faint appearance, astronomers classify NGC 1052-DF2 as an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015. This large, fuzzy-looking galaxy is so diffuse that they can clearly see distant galaxies behind it.
NGC 1052-DF2 is not well-formed: it does not look like a typical spiral galaxy, but it does not look like an elliptical galaxy either.
Based on the colors of its globular clusters, the galaxy is about 10 billion years old. However, even the globular clusters are strange: they are twice as large as typical groups of stars.
All of these oddities pale in comparison to the weirdest aspect of NGC 1052-DF2: it contains at least 400 times less dark matter than scientists predict for a galaxy of its mass, and possibly none at all.
“NGC 1052-DF2 is an oddity, even among this unusual class of galaxy,” said team member Shany Danieli, a graduate student at Yale University.
“Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy,” said team leader Dr. Pieter van Dokkum, also from Yale University.
“For decades, we thought that galaxies start their lives as blobs of dark matter. 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. NGC 1052-DF2 challenges the standard ideas of how we think galaxies form.”
The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
“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 team member Dr. Allison Merritt, of Yale University and the Max Planck Institute for Astronomy.
“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,” Dr. van Dokkum added.
“There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown,” Dr. Merritt said.
Meanwhile, the astronomers already have some ideas about how to explain the missing dark matter in NGC 1052-DF2.
This galaxy is a member of the NGC 1052 group of galaxies, which is dominated by the giant elliptical galaxy NGC 1052. Galaxy formation is turbulent and violent, and the team suggests that 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 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 are speculative, however, and don’t explain all of the characteristics of the observed galaxy,” the researchers said.
To find an explanation, the scientists are already hunting for more dark-matter deficient galaxies as they analyze Hubble images of 23 ultra-diffuse galaxies — three of which appear to be similar to NGC 1052-DF2.
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Pieter van Dokkum et al. 2018. A galaxy lacking dark matter. Nature 555, 629-632; doi: 10.1038/nature25767
