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Unusual galaxies defy dark matter theory

(27 March 2019 - WM Keck Observatory) 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.

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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. (courtesy: NASA, ESA, and P. van Dokkum (Yale University))

“If there’s one object, you always have a little voice in the back of your mind saying, ‘but what if you’re wrong?’ Even though we did all the checks we could think of, we were worried that nature had thrown us for a loop and had conspired to make something look really special whereas it was really something more mundane,” said team leader Pieter van Dokkum, Sol Goldman Family Professor of Astronomy at Yale University.

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 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.

“The fact that we’re seeing something that’s just completely new is what’s so fascinating,” said Danieli, who first spotted the galaxy about two years ago. “No one knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object, whether it’s a planet, a star, or a galaxy, that no one knew about or even thought about.”

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 were dark matter in DF2, the clusters would be moving much faster.

“KCWI is unique because of the combination of its large survey area,” said lead author Danieli. “The instrument not only allows us to see the whole galaxy at once, its high spectral resolution also enables us to measure the mass accurately. There is no other instrument in the world that has those two properties!”

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.

“Discovering a second galaxy with very little to no dark matter is just as exciting as the initial discovery of DF2,” said van Dokkum, who is the lead author on the DF4 paper. “This means the chances of finding more of these galaxies are now higher than we previously thought. Since we have no good ideas for how these galaxies were formed, I hope these discoveries will encourage more scientists to work on this puzzle.”

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

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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. (courtesy: 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 to 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.

“It was a little stressful at times,” said van Dokkum. “On one hand, this is how the scientific process is supposed to work; you see something interesting, other people disagree, you obtain new data, and in the end you learn more about the universe. On the other hand, although the majority of the critiques were constructive and polite, not all of them were. Every time a new critique came out we had to scramble and figure out if we had missed something.”

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.

“We hope to next find out how common these galaxies are and whether they exist in other areas of the universe,” said Danieli. “We want to find more evidence that will help us understand how the properties of these galaxies work with our current theories. Our hope is that this will take us one step further in understanding one of the biggest mysteries in our universe – the nature of dark matter.”

About KCWI

The Keck Cosmic Web Imager (KCWI) is designed to provide visible band, integral field spectroscopy with moderate to high spectral resolution formats and excellent sky-subtraction. The astronomical seeing and large aperture of the telescope will enable studies of the connection between galaxies and the gas in their dark matter halos, stellar relics, star clusters, and lensed galaxies. Support for this project was provided by The Heising-Simons Foundation, Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, and other Friends of Keck Observatory.

About LRIS

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I. Since it has been commissioned it has seen two major upgrades to further enhance its capabilities: addition of a second, blue arm optimized for shorter wavelengths of light; and the installation of detectors that are much more sensitive at the longest (red)wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in2011 for research determining that the universe was speeding up in its expansion.

About W. M. Keck Observatory

The W. M. Keck Observatory telescopes are the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.