Smallest Galaxy Ever Discovered Bolsters Dark Matter Theory

This image shows a prediction for the distribution of thousands of dark matter clumps called halos within about 1 million light years of the Milky Way galaxy, The disk of the Milky Way sits within the white region at the center. The zoomed-in image is of Segue 2 lying within one of these halos.
Credit: Garrison-Kimmel, Bullock (UCI)


This week, astronomers announced that they have managed to weigh a minuscule galaxy that breaks the record for the smallest ever detected and may help shed light on mysterious dark matter.

With only 1000 stars huddled together, this galactic featherweight dubbed Segue 2 has just enough mass  for dark matter to act as a glue keeping it together, according to a new study published this week in The Astrophysical Journal. (See also Dark Matter Galaxy Detected)

“Finding a galaxy as tiny as Segue 2 is like discovering an elephant smaller than a mouse,” said UC Irvine cosmologist and study co-author James Bullock in a statement.

Using the combined power of the twin 10 meter Keck telescopes perched on top of Mauna Kea in Hawaii, researchers were able to determine the weight of over a dozen of the most massive stars within this tiny galaxy. They were astonished to find that they were all ten times lighter than expected. A halo of surrounding dark matter–  the invisible material detectible only by the effects of its gravitational pull that scientists think makes up more than 80 percent of the mass in the universe – is what keeps Segue 2 together and qualifies it as a bonafide galaxy and not just a humdrum star cluster.

First glimpsed in 2009 by the Sloan Digital Sky Survey, Segue 2 turns out to be one of the faintest known galaxies too, shining with the power output of only 900 suns. To put that into perspective, our own Milky Way galaxy shines 20 billion times brighter. (Related Galaxy “Crumbs” Found in Milky Way—Proof of Recent Feeding)

Artist simulation of dark matter halo around the Milky Way galaxy.'s own backyard.  Credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI)
Artist simulation of dark matter halo around the Milky Way galaxy.’s own backyard. Credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI)

Small-fry galaxies like Segue 2 have been theorized to exist but have eluded detection until now. This discovery now bolsters our understanding of galactic formation across the universe and suggests that dark matter is indeed clumped into halos as predicted.

But co-author James Bullock and his team believe that tiny Segue 2 may in fact be just one of multitudes of tiny satellite galaxies buzzing around our Milky Way waiting to be discovered.

It could very well be only “a tip-of-the-iceberg observation, with perhaps thousands more very low-mass systems [dwarf galaxies] orbiting just beyond our ability to detect them,” he added. (See also Dying Galaxy Found Bleeding Out Into Space)


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Meet the Author
Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.