Unusual Galaxies Defy Dark Matter Theory

Two new studies confirm that galaxies lacking dark matter do in fact exist.  (Image: via   pixabay  /  CC0 1.0)
Two new studies confirm that galaxies lacking dark matter do in fact exist. (Image: via pixabay / CC0 1.0)

After drawing both praise and skepticism, the team of astronomers who discovered NGC 1052-DF2 — the very first known galaxy to contain little to no dark matter — are back with stronger evidence about its bizarre nature. Dark matter is a mysterious, invisible substance that typically dominates the makeup of galaxies; finding an object that’s missing dark matter is unprecedented and came as a complete surprise.

Now, van Dokkum’s team has not one, but two, new studies supporting their initial observations, demonstrating that dark matter is in fact separable from galaxies. Team leader Pieter van Dokkum, Sol Goldman Family Professor of Astronomy at Yale University, said:

NGC 1052-DF2 IS AN ULTRA-DIFFUSE GALAXY, A RELATIVELY NEW TYPE OF GALAXY THAT WAS FIRST DISCOVERED IN 2015 WITH THE DRAGONFLY TELEPHOTO ARRAY. FOLLOW UP OBSERVATIONS AT W. M. KECK OBSERVATORY REVEALED THAT THIS BIZARRE GALAXY CONTAINS LITTLE TO NO DARK MATTER. (CREDITS: NASA, ESA, AND P. VAN DOKKUM (YALE UNIVERSITY))

NGC 1052-DF2 is an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015 with the Dragonfly Telephoto Array. Follow-up observations at W. M. Keck observatory revealed that this bizarre galaxy contains little to no dark matter. (Image: NASA, ESA, and P. Van Dokkum (Yale University))

Team members include Roberto Abraham, Professor of Astronomy and Astrophysics at the University of Toronto; Aaron Romanowsky, Associate Professor of Physics and Astronomy at San Jose State University; Charlie Conroy, Professor of Astronomy at Harvard University; and Shany Danieli, a graduate student at Yale University. Danieli, who first spotted the galaxy about two years ago, said:

In the first study, the team confirmed their initial observations of NGC 1052-DF2, or DF2 for short, which show dark matter is practically absent in the galaxy. Using W. M. Keck Observatory’s Keck Cosmic Web Imager (KCWI), they gathered more precise measurements and found that the globular clusters inside the galaxy are indeed moving at a speed consistent with the mass of the galaxy’s normal matter.

If there was dark matter in DF2, the clusters would be moving much faster. Lead author Danieli said:

In the second study, the team used Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) to find another galaxy devoid of dark matter, named NGC 1052-DF4, or DF4 for short. Van Dokkum, who is the lead author on the DF4 paper, said:

The team’s results are published in The Astrophysical Journal Letters; the first study appears in today’s issuewhile the second study appears in the March 20th issue.

A survey image taken with the Dragonfly Telephoto Array shows objects within the field of the elliptical galaxy NGC 1052 (center). Among these objects are DF2 (bottom left) and DF4 (top right); both are dark matter-deficient galaxies that are similar in size, luminosity, morphology, globular cluster population, and velocity dispersion. (CREDIT: P. VAN DOKKUM (YALE UNIVERSITY)/STScI/ACS)

A survey image taken with the Dragonfly Telephoto Array shows objects within the field of the elliptical galaxy NGC 1052 (center). Among these objects are DF2 (bottom left) and DF4 (top right); both are dark matter-deficient galaxies that are similar in size, luminosity, morphology, globular cluster population, and velocity dispersion. (CREDIT: P. VAN DOKKUM (YALE UNIVERSITY)/STScI/ACS)

Like DF2, DF4 belongs to a relatively new class of galaxies called ultra-diffuse galaxies (UDGs). They are as large as the Milky Way but have between 100 and 1000 times fewer stars, making them appear fluffy and translucent, therefore difficult to observe.

Ironically, the lack of dark matter in these UDGs strengthens the dark matter theory. It proves that dark matter is a substance that is not coupled to “normal” matter, as both can be found separately. The discovery of these galaxies is difficult to explain in theories that change the laws of gravity on large scales as an alternative to the dark matter hypothesis.

This shocking discovery drew some criticism when the team first announced their results in March of 2018. van Dokkum said:

Van Dokkum says he’s proud of his team for pulling together in those tough moments. Their hard work has paid off, with the universe cooperating and giving more reason to look for other UDGs like DF2 and DF4.

Danieli is leading a wide area survey with the Dragonfly Telephoto Array (DTA) to look for more examples in a systematic way, then observe candidates again using the Keck telescopes. Danieli said:

Provided by: W. M. Keck Observatory [Note: Materials may be edited for content and length.]

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