Strange Dark Galaxy Puzzles Astrophysicists
Quanta Magazine said:Among the thousand-plus galaxies in the Coma cluster, a massive clump of matter some 300 million light-years away, is at least one — and maybe a few hundred — that shouldn’t exist.
Dragonfly 44 is a dim galaxy, with one star for every hundred in our Milky Way. But it spans roughly as much space as the Milky Way. In addition, it’s heavy enough to rival our own galaxy in mass, according to results published in the Astrophysical Journal Letters at the end of August. That odd combination is crucial: Dragonfly 44 is so dark, so fluffy, and so heavy that some astronomers believe it will either force a revision of our theories of galaxy formation or help us understand the properties of dark matter, the mysterious stuff that interacts with normal matter via gravity and not much else. Or both.
...
In these theories, clumps of dark matter seed the universe with light. First, clouds of dark matter coalesce into relatively dense dark-matter haloes. Then gas and fragments of other galaxies, drawn by the halo’s gravity, collect at the center. They spin out into a disk and collapse into luminous stars to form something we can see through telescopes. The whole process seems to be reasonably predictable for big galaxies such as our Milky Way. Having measured either a galaxy’s dark-matter halo or its assortment of stars, you should be able to predict the other to within a factor of two.
“It’s not just dogma. It’s basically that there are no exceptions that we knew of,” said Jeremiah Ostriker, an astrophysicist at Columbia University.
After Abraham and van Dokkum realized that they appeared to be looking at 47 exceptions, they did a search through the literature. They found that similar fuzzy blobs have been on the edge of discovery since the 1970s. Van Dokkum thinks astronomy’s transition from photographic plates — which were perhaps better suited to picking up extended, diffuse objects — to modern digital sensors may actually have hid them from further attention.
...