From Gemini and Keck: “Dark Matter is a No Show in Ghostly Galaxy”


Gemini Observatory
Gemini Observatory

Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft) above sea level, with Subaru and IRTF (NASA Infrared Telescope Facility). Vadim Kurland

Keck Observatory

Science Contacts:

Pieter van Dokkum
Astronomy Department
Yale University
Phone: 203-432-5048

Shany Danieli
Astronomy Department
Yale University
Phone: 857-919-3674

Media Contacts:

Mari-Ela Chock
W.M. Keck Observatory
Phone: 808-554-0567

Jasmin Silva
Gemini Observatory
Desk: 808 974-2575

Composite color image of NGC1052-DF2 constructed from observations using the Gemini Multi Object Spectrograph (GMOS) on Gemini North on Hawai‘i’s Maunakea. The ultra-diffuse galaxy was observed using deep imaging in two filters (g’ and i’). Image credit: Gemini Observatory/NSF/AURA/Keck/Jen Miller.


Left: The ultra-diffuse galaxy is swarming with globular clusters, which hold the key to understanding this mysterious object’s origin and mass.
Right: A closer look at one of the globular clusters within the galaxy, which are all much brighter than typical, the brightest emitting almost as much light as the brightest within the Milky Way. The spectrum, obtained by Keck Observatory shows the absorption lines used to determine the velocity of this object. Ten clusters were observed, providing the information needed to determine the mass of the galaxy, revealing its lack of dark matter. Image credit: Gemini Observatory/NSF/AURA/Keck/Jen Miller/Joy Pollard.

Astronomers using data from the Gemini and W. M. Keck Observatories in Hawai‘i have encountered a galaxy that appears to have almost no dark matter. Since the Universe is dominated by dark matter, and it is the foundation upon which galaxies are built, “…this is a game changer,” according to Principal Investigator Pieter van Dokkum of Yale University.

Galaxies and dark matter go hand in hand; you typically don’t find one without the other. So when researchers uncovered a galaxy, known as NGC1052-DF2, that is almost completely devoid of the stuff, they were shocked.

“Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy,” said lead author Pieter van Dokkum of 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. NGC1052-DF2 challenges the standard ideas of how we think galaxies form.”

The research, published in the March 29th issue of the journal Nature, amassed data from the Gemini North and W. M. Keck Observatories, both on Maunakea, Hawai‘i, the Hubble Space Telescope, and other telescopes around the world.

NASA/ESA Hubble Telescope

Given its large size and faint appearance, astronomers classify NGC1052-DF2 as an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015. Ultra-diffuse galaxies are surprisingly common. However, no other galaxy of this type yet-discovered is so lacking in dark matter.

“NGC1052-DF2 is an oddity, even among this unusual class of galaxy,” said Shany Danieli, a Yale University graduate student on the team.

To peer even deeper into this unique galaxy, the team used the Gemini Multi Object Spectrograph (GMOS) to capture detailed images of NGC1052-DF2, assess its structure, and confirm that the galaxy had no signs of interactions with other galaxies.

“Without the Gemini images dissecting the galaxy’s morphology we would have lacked context for the rest of the data,” said Danieli. “Also, Gemini’s confirmation that NGC1052-DF2 is not currently interacting with another galaxy will help us answer questions about the conditions surrounding its birth.”

Van Dokkum and his team first spotted NGC1052-DF2 with the Dragonfly Telephoto Array, a custom-built telescope in New Mexico that they designed to find these ghostly galaxies.

U Toronta Dragon Fly Telescope Array housed in New Mexico

NGC1052-DF2 stood out in stark contrast when comparisons were made between images from the Dragonfly Telephoto Array and the Sloan Digital Sky Survey (SDSS).

SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

The Dragonfly images show a faint “blob-like” object, while SDSS data reveal a collection of relatively bright point-like sources.

In addition to the Gemini observations, to further assess this inconsistency the team dissected the light from several of the bright sources within NGC1052-DF2 using Keck’s Deep Imaging Multi-Object Spectrograph (DEIMOS) and Low-Resolution Imaging Spectrometer (LRIS), identifying 10 globular clusters. These clusters are large compact groups of stars that orbit the galactic core.

Keck/DEIMOS on Keck 2


The spectral data obtained on the Keck telescopes revealed that the globular clusters were moving much slower than expected. The slower the objects in a system move, the less mass there is in that system. The team’s calculations show that all of the mass in the galaxy could be attributed to the mass of the stars, which means there is almost no dark matter in NGC1052-DF2.

“If there is any dark matter at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can account for all of the mass, and there doesn’t seem to be any room for dark matter.”

The team’s results demonstrate that dark matter is separable from galaxies. “This discovery shows that dark matter is real – it has its own separate existence apart from other components of galaxies,” said van Dokkum.

NGC1052-DF2’s globular clusters and atypical structure has perplexed astronomers aiming to determine the conditions this galaxy formed under.

“It’s like you take a galaxy and you only have the stellar halo and globular clusters, and it somehow forgot to make everything else,” van Dokkum said. “There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown.”

However, researchers do have some ideas. NGC1052-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 van Dokkum suggests that the growth of the fledgling massive galaxy billions of years ago perhaps played a role in NGC1052-DF2’s dark-matter deficiency.

Another idea is that a cataclysmic event within the oddball galaxy, such as the birth of myriad massive stars, swept out all the gas and dark matter, halting star formation.

These possibilities are speculative, however, and don’t explain all of the characteristics of the observed galaxy, the researchers add.

The team continues the hunt for more dark-matter-deficient galaxies. They are analyzing Hubble images of 23 other diffuse galaxies. Three of them appear to share similarities with NGC1052-DF2, which van Dokkum plans to follow up on in the coming months at Keck Observatory.

“Every galaxy we knew about before has dark matter, and they all fall in familiar categories like spiral or elliptical galaxies,” van Dokkum said. “But what would you get if there were no dark matter at all? Maybe this is what you would get.”

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The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometer and world-leading laser guide star adaptive optics systems. Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Today Keck Observatory is supported by both public funding sources and private philanthropy. As a 501(c)3, the organization is managed by the California Association for Research in Astronomy (CARA), whose Board of Directors includes representatives from the California Institute of Technology and the University of California, with liaisons to the board from NASA and the Keck Foundation.

Keck UCal

Gemini/North telescope at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet


Gemini’s mission is to advance our knowledge of the Universe by providing the international Gemini Community with forefront access to the entire sky.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai’i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in six partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country’s contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, and the Brazilian Ministério da Ciência, Tecnologia e Inovação. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.