Just beyond the Milky Way, astronomers have found an extremely dim dwarf galaxy that appears to have just a few hundred stars, but is surprisingly massive. Researchers say the galaxy, called Segue 1, must be packed with mysterious dark matter in order to give it such bulk.
Dark matter has never been directly detected, and its presence can only be deduced: Although dark matter doesn’t emit or absorb light, scientists can measure its gravitational effect on ordinary matter and believe it makes up about 85 percent of the total mass in the universe. Dark matter is thought to play a crucial role in galaxy formation, perhaps by contributing to the clumps that stimulate star formation in a budding galaxy and by contributing to the overall matter of a galaxy that allows it to lure other matter and galaxies inward in a growth-by-merger process [SPACE.com].
The discovery of Segue 1, which will be described in a forthcoming issue of Astrophysical Journal, gives astronomers a new venue in which to study dark matter. The object is one of a larger group of recently discovered galaxies where dark matter seems to outweigh ordinary matter by a factor of 100 or 1000. In the Milky Way, dark matter is thought to outweigh luminous matter by a factor of 10. “These galaxies are basically invisible,” says [study co-author] Beth Willman…. “We didn’t know these kinds of galaxies existed until a couple of years ago” [New Scientist].
The game changed with broad surveys like the Sloan Digital Sky Survey, which scanned the skies for areas where stars appeared to be slightly more dense than usual. Researchers are now wondering if these dim, dwarf galaxies are, in fact, the norm: “For every bright galaxy we can detect in the sky, there are likely hundreds of ultra-faint dwarf galaxies surrounding them that we are unable to detect with current technology,” [said co-author] James Bullock…. “It seems that these nearly-invisible galaxies are likely the most common galaxies in the universe” [Cosmos].
Image: M. Geha
September 22nd, 2008 at 10:04 am
“For every bright galaxy we can detect in the sky, there are likely hundreds of ultra-faint dwarf galaxies surrounding them that we are unable to detect with current technology,” [said co-author] James Bullock…. “It seems that these nearly-invisible galaxies are likely the most common galaxies in the universe.”
This doesn’t make any sense. If there are really 100 of these faint galaxies for every known galaxy, you’ve just increased the mass of the universe by 100 times. A halo of dim galaxies, “undetectable by current technology” and surrounding every bright galaxy, wouldn’t that pretty much account for all the “dark matter” right there? This really doesn’t make any sense to me. They’re actually saying they’ve been off by a factor of 100 for the number of galaxies out there, because they just weren’t looking closely enough?
September 22nd, 2008 at 9:46 pm
Why is that so hard to believe? Scientists have known for some time the the value of the measurable mass doesn’t account for the value of mass deduced by experiment and observations of the movement of galaxies. We know there’s a lot more out there than we can see.
Maybe not because we weren’t looking closely enough, but because for a long time we didn’t have the technology to look in the right way, and also because we didn’t know what we were looking for.