Galaxies aren’t just collections of stars. They also have to have a halo of Dark Matter cementing them together to qualify for that distinction. Otherwise any cluster would make the grade. There is an interesting new idea on how to explain Dark Matter making the rounds this week. Meanwhile, astronomers using the Keck Observatory have found what they believe is by far the tiniest galaxy ever seen, made up of only 1000 stars or so, that orbits the Milky Way just past the edge.
Astronomy Mag — “Finding a galaxy as tiny as Segue 2 is like discovering an elephant smaller than a mouse,” said James Bullock from UC-Irvine. Astronomers have been searching for years for this type of dwarf galaxy, long predicted to be swarming around the Milky Way. Their inability to find any, he said, “has been a major puzzle, suggesting that perhaps our theoretical understanding of structure formation in the universe was flawed in a serious way.”Segue 2’s presence as a satellite of our home galaxy could be “a tip-of-the-iceberg observation, with perhaps thousands more very low-mass systems orbiting just beyond our ability to detect them,” Bullock said.
Image Caption — This image shows a standard prediction for the dark matter distribution within about 1 million light years of the Milky Way galaxy, which is expected to be swarming with thousands of small dark matter clumps called `halos’. The scale of this image is such that the disk of the Milky Way would reside within the white region at the center. Until now, there was no observational evidence that dark matter actually clumps this way, raising concerns that our understanding of the cosmos was flawed in a fundamental way. Observations of the ultra-faint galaxy Segue 2 (zoomed image) have revealed that it must reside within such a tiny dark matter halo, providing possibly the first observational evidence that dark matter is as clumpy as long predicted. // Garrison-Kimmel, Bullock (UCI).