Astronomers Discover Most Dark Matter-Dominated Galaxy in Universe

September 18, 2008,

Segue 1 is 50 times dimmer than the star cluster pictured above but is 1000 times more massive, meaning most of its mass must be made up of dark matter. (Credit: Sloan Digital Sky Survey)
( -- A team led by a Yale University astronomer has discovered the least luminous, most dark matter-filled galaxy known to exist.

The galaxy, called Segue 1, is one of about two dozen small satellite galaxies orbiting our own Milky Way galaxy. The ultra-faint galaxy is a billion times less bright than the Milky Way, according to the team's results, to be published in an upcoming issue of The Astrophysical Journal (ApJ).

But despite its small number of visible stars, Segue 1 is nearly a thousand times more massive than it appears, meaning most of its mass must come from dark matter, according to the researchers.

"I'm excited about this object," said Marla Geha, an assistant professor of astronomy at Yale and the paper's lead author. "Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass."

Geha, along with her colleague Josh Simon at the California Institute of Technology, has observed about half of the dwarf satellite galaxies that orbit the Milky Way. These objects are so faint and contain so few stars that at first they were thought to be globular clusters – tightly bound star clusters that also orbit our host galaxy. But by analyzing the light coming from the objects using the Keck telescope in Hawaii, Geha and Simon showed that these objects are actually galaxies themselves, albeit very dim ones.

Looking only at the light emitted by these ultra-faint galaxies, Geha and her colleagues expected them to have correspondingly low masses. Instead, they discovered that they are between 100 and 1000 times more massive than they appear. Invisible dark matter, she said, must account for the difference.

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. Finding ultra-faint galaxies like Segue 1, which is so rife with dark matter, provides clues as to how galaxies form and evolve, especially at the smallest scales.

"These dwarf galaxies tell us a great deal about galaxy formation," Geha said. "For example, different theories about how galaxies form predict different numbers of dwarf galaxies versus large galaxies. So just comparing numbers is significant."

It's only recently that astronomers have discovered just how prevalent these dwarf satellite galaxies are, thanks to projects like the Sloan Digital Sky Survey, which imaged large areas of the nighttime sky in greater detail than ever before. In the past two years alone, the number of known dwarf galaxies orbiting the Milky Way has doubled from the dozen or so brightest that were discovered during the first half of the twentieth century.

Geha predicts astronomers will find even more as they continue to sift through new data. "The galaxies I now consider bright used to be the least luminous ones we knew about," she said. "It's a totally new regime. This is a story that's just unfolding."

Provided by Yale University

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2.6 / 5 (5) Sep 18, 2008
2 / 5 (4) Sep 18, 2008 dust...
4.1 / 5 (7) Sep 19, 2008
If that galaxy was filled with dust and gas that weigh 100 to a 1000 times more than the galaxy, instead of of dark matter, you'd expect the gas and dust to contract and form new stars.
4 / 5 (5) Sep 19, 2008
Yes. dust. We can barely detect rocky planets orbiting stars in our galaxy...
How do they know that most of the mass isn't made of debris, asteroids, and other non light sources??? Maybe star formation wasn't popular in that galaxy :-)

All debris like dust and planets and asteroids are associated with some star, these objects (including our dear Earth) is a debris from the formation of sun. So, if you don't see stars, i.e. stars don't exist then these debris are very unlikely to exist.

Also, the total mass of all the planets and asteroids and comets and everything in the solar system add up to 0.14% of the mass of solar system, the rest 99.86% of mass is that of the SUN! So, all these debris contribute very little to the mass of the solar system, and hence their contribution to the mass of galaxy is negligible.

So, even if unobserved visible massive bodies like planets etc. exist, their contribution is near to zero.

Better get used to dark mass, it exists.
4.3 / 5 (6) Sep 19, 2008
So they found the mass of Segue 1 to be 100 to 1000 times the "luminous mass". But how did they come to this conclusion? Which method was used to measure the total mass, luminous plus non-luminous?
Without that bit of information, it's just another non-falsifiable story.

Read carefully, in the article they mention that mass is estimated by gravitational effect of a body on the neighbouring bodies. This is the most accurate and correct measure of mass.

Get used to the idea of dark matter, it exists!
4 / 5 (4) Sep 19, 2008
Deducing mass from speed for objects far out from the disk of the galaxy is fraught with complications because ordinary Newtonian gravity cannot account for the actual behaviour of objects in this region. (Galaxy Rotation problem). They should have offered some description of the method they used - and preferably a detailed description. Those that have access to the Astrophysical Journal will presumably get that. The ordinary Physorg reader will have to pay for it (often for the second time since most research is, I'm guessing, publicly funded.)
4.5 / 5 (4) Sep 19, 2008
they also detect slight gravitational lensing through statistical analysis. This is useful in determining the amount of mass in the region. Dust absorbs light. Dark matter doesn't. That's how they're able to eliminate dust as a possible explanation. Its a real prickly problem as its only observed in other galaxies and it doesn't seem to be consistent from one galaxy to another.
3.7 / 5 (3) Sep 19, 2008
I may be showing a lot of my ignorance here .. but.. How do we know that the Milky Way is not surrounded by a field, force, belt or whatever that might effect any study or observation we make of other galaxies? Our Solar System is surrounded by belts and fields and resides in a "bubble" of "clean" space, right? Could we be looking through a distorted lens?
4.4 / 5 (7) Sep 19, 2008
The paper on which this press release was based can be found at arXiv:0809.2781v1 In it, the authors state that two different techniques were used to derive this objects mass, and that no neutral hydrogen has been found to date. Also, this galaxy was found to be the best candidate dwarf galaxy to search for signals of dark matter annihilation, due to its high mass-to-light ratio and its distance from Earth. No doubt, observations with the Fermi Gamma-ray Space Telescope will be undertaken ASAP. Figure 6 of this paper contains a fascinating diagram that shows(among other things) a near constant mass of a half-dozen of these objects with a wide range of luminosities, with Segue 1 being least luminous. This mass-luminosity relationship surely constrains models of how these objects were formed & what role dark matter plays in this curious relationship.
3.7 / 5 (3) Sep 19, 2008
@ smiffy, the paper referenced above is located at the site & it's free to all. Check it out.
3.3 / 5 (3) Sep 19, 2008
@ smiffy & gmurphy, the paper describes their 2 techniques used to estimate mass as follows: 1)using the "mass follows light relationship" the team measured the "velocity dispersion averaged over the projected radius" to derive Segue 1's mass & 2) "individual stellar velocity measurements" were used to derive mass. It was noted that both mass estimates agreed within errors for Segue 1. I didn't see any mention of weak lensing used to derive mass, although the total mass of this object is probably too low for this method to be practical. I look forward to gamma-ray observations with Fermi and other gamma-ray space telescopes to search for signals of DM annihilation to provide further constraints on DM content & distribution. This object & other low-luminosity dwarf galaxies will be prime targets( among others) in the search for DM annihilation signals.
not rated yet Sep 19, 2008
@ xyz. Thanks a lot and please pardon my ignorance of the Archive site.
3.7 / 5 (3) Sep 19, 2008
In the 2006 discovery paper by V. Belokurov et al (arXiv:astro-ph/0608448v1), the contours for several new dwarf galaxies, including Segue 1, appear distorted or elongated (see Fig 5 in paper), most likely due to gravitational interaction with the Milky Way. Given Segue 1's estimated distance of only 23 kiloparsecs, gravity has probably already stripped this galaxy of its gas (hence no detection of neutral hydrogen by radio telescopes) and distorted its shape. Tidal stripping and gravitational distortion appears common among these ultra-faint Milky Way dwarf galaxies. Although the above paper finds Segue 1 to be a probable globular cluster (associated with the Sagittarius dSph), the newest paper mentioned in my post above challenges this assertion. These dark matter dominated systems are sure to garner much attention among astronomers over the next few years.
4 / 5 (2) Sep 21, 2008
Yes. dust. We can barely detect rocky planets orbiting stars in our galaxy...
How do they know that most of the mass isn't made of debris, asteroids, and other non light sources??? Maybe star formation wasn't popular in that galaxy :-)
Dust in large quantities is easily detectable, that's what nebulae are.

... and if it was dust that accounted for 99.9% of these galaxies, it would be a more astounding discovery than dark matter, since the matter would have to be something other than Hydrogen.
5 / 5 (1) Sep 22, 2008
I could add speculation that it could be full of warm neutrinos. And there will be no anihalation signature to detect. Probably black holes and planets are unlikely with so few stars.
1 / 5 (1) Sep 22, 2008
@Graeme, excellent point & one well worthy of discussion. Failure to detect gamma ray annihilation from these faint, DM dominated dwarf galaxies, as well as from our own galaxy would serve to constrain models of just what constitutes dark matter. Warm neutrinos (hot DM), axions,sneutrinos or other postulated particles are certainly still possibilities as to the true nature of DM. I also agree that black holes, planets, dust, etc have been intensely sought out in previous searches but seem not to fit the bill.Still, these high-DM to luminosity dwarfs could be key players in nailing down the true nature of dark matter.

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