New York, August 27, 2016: An international team of astronomers has found a massive galaxy — about the size of the Milky Way — that consists almost entirely of dark matter.
The galaxy, Dragonfly 44, is located in the nearby Coma constellation and had been overlooked until last year because of its unusual composition. It is a diffuse “blob” about the size of the Milky Way, but with far fewer stars, the researchers said.
“Very soon after its discovery, we realised this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together,” said lead author Pieter van Dokkum from Yale University in New Haven, Connecticut.
Dragonfly 44’s mass is estimated to be one trillion times the mass of the Sun, which is similar to the mass of the Milky Way. However, only one-hundredth of one percent of that is in the form of stars and “normal” matter.
The other 99.99 percent is in the form of dark matter — a hypothesised material that remains unseen but may make up more than 90 percent of the universe, the researchers reported in a paper published in the Astrophysical Journal Letters.
Van Dokkum’s team was able to get a good look at Dragonfly 44 thanks to the W.M. Keck Observatory and the Gemini North telescope, both in Hawaii.
Astronomers used observations from Keck, taken over six nights, to measure the velocities of stars in the galaxy.
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They used the eight-metre Gemini North telescope to reveal a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way galaxy.
Star velocities are an indication of the galaxy’s mass, the researchers noted. The faster the stars move, the more mass its galaxy will have.
“Amazingly, the stars move at velocities that are far greater than expected for such a dim galaxy. It means that Dragonfly 44 has a huge amount of unseen mass,” co-author Roberto Abraham of the University of Toronto, explained.