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.”
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.
The team used Keck Observatory’s Keck Cosmic Web Imager () 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.