Astronomers have followed a faint, cosmic trail of gas to a third galaxy that has no dark matter.
In a new study in The Astrophysical Journal, a team of Yale astronomers reports on a dwarf galaxy located45 million light-years from Earth — called NGC 1052-DF9 — that appears to have formed in a straight line with nine other galaxies.
Two of those other galaxies, DF2 and DF4, were previously shown to lack dark matter — an invisible, theorized material that gives shape to the universe and is thought by most astronomers to be essential to galaxy formation.
Now, DF9 has joined the no-dark-matter club.
"A line of galaxies lacking dark matter has never been seen before," said Michael Keim, a Ph.D. student in astrophysics in Yale's Graduate School of Arts and Sciences and first author of the new study. "The discoveryprovides some of the strongest evidence yet that these galaxies formed through an extreme and previously unseen process and offers a rare new window into the nature of dark matter itself."
Keim's advisor, Yale astronomer Pieter van Dokkum, led the original studies that analyzed DF2 and DF4, using data from the Hubble Space Telescope. Van Dokkum, the Sol Goldman Family Professor of Astronomy and professor of physics in Yale's Faculty of Arts and Sciences, is co-author of the new study.
During his doctoral work with van Dokkum, Keim found DF9 — which had been misidentified as a supermassive black hole — and proposed a thorough analysis with W.M. Keck Observatory's Cosmic Web Imager, in Hawaii, which is designed specifically to study faint starlight such as the light emitted by DF9.
The researchers measured the motions of stars within DF9 to determine its mass. They found that DF9 has the mass of 100 million suns — which is consistent with the expected amount of visible matter in a galaxy of its size — and nothing else. If DF9 also had the expected amount of dark matter, its mass would be equal tomore than 10 billion suns.
DF9's lack of dark matter strongly suggests that DF2, DF4, and DF9 formed together in the same violent event, such as a high-speed collision between galaxies, Keim said. In this scenario, the collision would have separated out gas from the galaxies' dark matter — and that gas went on to form new galaxies in a linear trail.
"Up until now it was assumed galaxies formed within pools of dark matter called 'halos,'" Keim said. "This system shows that stars and galaxies can form outside of dark matter 'halos' in extreme events and indicates that dark matter is a physical substance that can act independently of normal matter or gas, challenging alternative theories that dark matter is gravity."
In this regard, the new study reaffirms van Dokkum's original work on DF2 and DF4, which also suggested that dark matter is a separate material.
The researchers are now conducting follow-up observations with other telescopes — including the new Mothra telescope co-founded by van Dokkum and University of Toronto astronomer Roberto Abraham — to search for any gas that was left behind after the initial galaxy collision.
Co-authors of the study are former Yale Ph.D. student Zili Shen, Yale postdoctoral researcher Imad Pasha, and Princeton astronomer Shany Danieli.
