Astronomers using the W. M. Keck Observatory and the Gemini North telescope have found a galaxy that consists almost entirely of dark matter, the mysterious and elusive substance thought to make up almost a quarter of the Universe.
The ultra-diffuse galaxy Dragonfly 44 and its immediate surroundings. The galaxy consists almost entirely of dark matter, and has a remarkable appearance: it is a low surface brightness, spheroidal object that is peppered with faint, compact sources. Image credit: Pieter van Dokkum / Roberto Abraham / Gemini Observatory / SDSS / AURA.
The galaxy in question, Dragonfly 44, is an ultra-diffuse galaxy located 300 million light-years away in the constellation Coma.
It was discovered a year ago by Yale University astronomer Dr. Pieter van Dokkum and colleagues using the Dragonfly Telephoto Array.
Upon further scrutiny, the astronomers realized the galaxy had to have more than meets the eye: it has so few stars that it quickly would be ripped apart unless something was holding it together.
To determine the amount of dark matter in this galaxy, they used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass.
The scientists then used the Gemini Multi-Object Spectrograph on the 8-m Gemini North telescope to reveal a halo of spherical clusters of stars around the galaxy’s core.
“Motions of the stars tell you how much matter there is. They don’t care what form the matter is, they just tell you that it’s there. In the Dragonfly galaxy stars move very fast,” Dr. van Dokkum said.
“So there was a huge discrepancy: using Keck Observatory, we found many times more mass indicated by the motions of the stars, then there is mass in the stars themselves.”
The mass of Dragonfly 44 is estimated to be a trillion times the mass of the Sun – very similar to the mass of our Milky Way Galaxy.
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.
The Milky Way has more than a hundred times more stars than Dragonfly 44.
Finding a galaxy with the mass of the Milky Way that is almost entirely dark was unexpected.
“We have no idea how galaxies like Dragonfly 44 could have formed,” said co-author Dr. Roberto Abraham, from the University of Toronto.
“The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”
“This has big implications for the study of dark matter,” Dr. van Dokkum said.
“It helps to have objects that are almost entirely made of dark matter so we don’t get confused by stars and all the other things that galaxies have.”
“The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study.”
The team’s findings have been accepted for publication in the Astrophysical Journal Letters (arXiv.org preprint).
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Pieter van Dokkum et al. 2016. A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44. ApJL, accepted for publication; arXiv: 1606.06291
