The discovery strengthens evidence for a rare and previously unseen process in which ordinary matter becomes separated from dark matter, offering astronomers a powerful new way to study one of the universe’s greatest mysteries.
Researchers believe the collision may have stripped gas away from its surrounding dark matter, allowing new galaxies to form from ordinary matter alone.
If the galaxy contained a typical amount of dark matter, astronomers would expect it to be about 100 times more massive.
“KCWI’s exceptionally high precision enabled us to measure DF9’s extraordinarily low mass with the accuracy needed to demonstrate its lack of dark matter,” said Keim.
ABOUT W. M. KECK OBSERVATORYThe W. M. Keck Observatory telescopes are among the most scientifically productive on Earth.
Findings strengthen evidence for a violent galactic collision that may have separated ordinary matter from dark matter
Maunakea, Hawaiʻi – Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island, have discovered the third known galaxy apparently lacking dark matter, part of a strange linear structure that may have formed during a violent collision between galaxies.
A close-up Hubble image of DF9 is shown beneath a wider view of the surrounding NGC 1052 region. Blue boxes highlight a line of related galaxies, including DF2 and DF4. Red outlines show where Keck Observatory’s KCWI instrument collected data, while yellow circles mark galaxy clusters whose motions have already been measured. Both images are oriented along the direction of the galactic structure. (Credit: Keim et al./DECaLS/HST).
The discovery strengthens evidence for a rare and previously unseen process in which ordinary matter becomes separated from dark matter, offering astronomers a powerful new way to study one of the universe’s greatest mysteries.
The galaxy, known as DF9, lies alongside two other unusual galaxies — DF2 and DF4 — which previously stunned astronomers because they appeared to contain little to no dark matter. New observations show that DF9 also lacks dark matter and is part of the same narrow line of faint, diffuse galaxies stretching across space.
“Almost every galaxy in the universe is dominated by dark matter. But DF2, DF4, and now DF9 appear to be extraordinary exceptions,” said Michael Keim, researcher and lead author of the study. “These findings provide some of the clearest evidence yet that these galaxies formed together in a violent event that separated ordinary matter from dark matter.”
The study, led by Yale University, is published today in The Astrophysical Journal.
A New Clue in the Dark Matter Mystery
Yale astronomers have played a central role in the discovery of dark matter-deficient galaxies since the first identification of DF2 and DF4 by astronomer Pieter van Dokkum and his team, who also used Keck Observatory observations to help confirm their unusual nature.
The discovery of DF9 strengthens the case that all three galaxies formed together during the same violent event, likely a high-speed collision between galaxies. Such a system has never been observed before and is reshaping astronomers’ understanding of how galaxies form.
Researchers believe the collision may have stripped gas away from its surrounding dark matter, allowing new galaxies to form from ordinary matter alone.
“The finding provides compelling evidence that dark matter behaves as a physical substance rather than the effect of an alternative theory of gravity, particularly at the dwarf-galaxy scale where those theories are most heavily debated,” added van Dokkum, co-author on the study.
Measuring the Invisible
The team used Keck Observatory’s Keck Cosmic Web Imager (KCWI) to measure the motions of stars inside DF9 by analyzing the light emitted across different wavelengths.
Those measurements revealed that DF9 has a mass of only about 100 million Suns, consistent entirely with the galaxy’s visible matter. If the galaxy contained a typical amount of dark matter, astronomers would expect it to be about 100 times more massive.
“KCWI’s exceptionally high precision enabled us to measure DF9’s extraordinarily low mass with the accuracy needed to demonstrate its lack of dark matter,” said Keim.
Building on these observations, future studies using both existing and upcoming observatories will enable the team to search for gas that may have been left behind in the collision and to constrain the gas content of the galaxies themselves.
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Science Contacts:
Michael Keim [email protected]
Pieter Van Dokkum [email protected]
Media Contact:
Meagan O’Shea [email protected]
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 enables studies of the connection between galaxies and the gas in their dark matter halos, stellar relics, star clusters, and lensed galaxies. KCWI covers the blue side of the visible spectrum; the instrument also features the Keck Cosmic Reionization Mapper (KCRM), extending KCWI’s coverage to the red side of the visible spectrum. The combination of KCWI-blue and KCRM provides simultaneous high-efficiency spectral coverage across the entire visible spectrum. Support for KCWI was provided by the National Science Foundation, Heising-Simons Foundation, and Mt. Cuba Astronomical Foundation. Support for KCRM was provided by the National Science Foundation and Mt. Cuba Astronomical Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaiʻi 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. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization 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 wish to recognize and acknowledge the very significant cultural role and reverence 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. For more information, visit: www.keckobservatory.org
