Saturday, December 5, 2020

Astronomers claimed galaxy was 98% dark matter. They were wrong.

Back in 2016, researchers claimed to have found a galaxy made almost completely of dark matter and almost no stars. Now, on closer examination, that claim has fallen apart.

The galaxy, Dragonfly 44 (DF44), belongs to a class of mysterious objects known as ultra-diffuse galaxies or UDGs. Researchers have debated since the 1980s whether these vast, dim objects have a low mass, like dwarf galaxies smeared across huge reaches of space, or more like heavy, Milky Way-style galaxies that seem dim for two reasons: because they have almost no stars, and because a huge fraction of their mass is dark matter found in the outer fringes of the galaxy, in so-called  dark matter haloes that emit no light. In a paper published in 2016 in The Astrophysical Journal Letters, scientists argued that DF44 was one of these galaxies with a big dark matter halo and few stars. They estimated its mass and found it was at least 98% dark matter. 

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