A group of astronomers — three of whom are from the Yale Astronomy Department — recently discovered a new galaxy comprised mostly of dark matter, a type of matter which does not give off traceable light or energy. The authors have published their findings in a Sept. 1 issue of The Astrophysical Journal Letters.
The team of astronomers focussed on the new galaxy discovered with the Dragonfly Telephoto Array, a telescope better suited for detecting dark matter. Dark matter makes up a relatively large part of the universe’s mass — about 27 percent. In comparison, galaxies and stars comprise nearly 5 percent. The only way people can observe dark matter is to analyze the gravitational effect it has on visible matter, according to the National Aeronautics and Space Administration.
“We suspected that this galaxy would have a lot of dark matter, but we were surprised to find that it is almost entirely made out of it. We did not know that galaxies as big and massive as the Milky Way could be composed almost entirely of dark matter,” said Pieter van Dokkum, first author of the study and professor in the Yale Department of Astronomy.
The galaxy, named Dragonfly 44, is classified as an ultra-diffuse galaxy. Ultra-diffuse galaxies, or UDGs, are substantive galaxies and are known for emitting low levels of brightness compared to other galaxies. Previously, astronomers didn’t know how to properly classify UDGs. They have been thought to be the product of smaller galaxies or dimmer stellar disks that expanded over time. They also could have resulted from the fastest spinning tails of dwarf galaxies, or from “failed” galaxies that couldn’t accumulate enough stars, according to the paper.
The group measured the stellar dynamics of Dragonfly 44 with the Deep Imaging Multi-Object Spectrograph, also known as DEIMOS, on the Keck II telescope in Hawaii. They then used the Gemini North telescope to measure the size of the spherical cluster of stars inside the galaxy.
The astronomers found that Dragonfly 44 was a galaxy with a large mass, size and globular cluster population, despite its low levels of visible light. The amount of globular clusters contributed to the galaxy’s large mass. The galaxy has a high ratio of mass to light and 98 percent of the galaxy is estimated to be dark matter.
Since the galaxy has a substantive amount of mass, it contributes to the theory that UDGs result from “failed” galaxies, instead of just being expanded versions of smaller galaxies. Unlike most galaxies, UDGs have low luminosity and do not display a classic bulge in their formation.
“This discovery brings up the question ‘How dark can a galaxy be?’ It will be interesting to see if there are dark, massive galaxies out there that have even fewer stars than Dragonfly 44 — or no stars at all. We’re also looking at examples of this galaxy that are closer to Earth; that would enable us to study them better, and also look for any signs of the elusive dark-matter particle,” van Dokkum said.
Van Dokkum said that he and his team have recently updated the Dragonfly Telephoto array, so they can study the sky about six times faster than before, and plan to use it to locate darker UDGs, possibly closer to Earth.
The study was a collaborative effort with researchers from the Yale Astronomy Department, the University of Toronto Department of Astronomy & Astrophysics, the University of California Observatories, the Harvard-Smithsonian Center for Astrophysics and the San José State University Department of Physics and Astronomy.