Companion Dwarf Galaxy Almost Invisible

Article written: 18 Sep , 2008
Updated: 24 Dec , 2015

A team of astronomers has discovered the least luminous, most dark matter-filled galaxy known to exist. The Segue 1 galaxy is one of about two dozen small satellite galaxies orbiting our own Milky Way. This is a very faint galaxy, a billion times less bright than the Milky Way. 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. “Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass,” said Marla Geha, an assistant professor of astronomy at Yale and lead author on a paper about Segue 1.

Geha and her colleagues have 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, the researchers determined these objects are actually galaxies, but just very faint.

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.”

Source: Yale University

6 Responses

  1. Jon Hanford says

    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.

  2. joe says

    Since these galaxies are orbiting our milky way they are already gravitationally involved. What kind of changes would there have to be before these galaxies begin to exchange gas and perhaps stars as we see happening elsewhere. Or is it that everything is balanced as is and the orbital parameters are static and distances are too extreme to allow a destructive interaction?

  3. Jon Hanford says

    @ joe, indeed 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. RetardedFishFrog says

    I’m glad to see they looked at hydrogen and ruled it out as the source of the extra gravity. That’s always my first suspect when astronomers think they’ve found dark matter. I’m not an expert, but I am a dark matter skeptic. With hydrogen ruled out, I think the next most obvious answer would be a dwarf super massive black hole. Possibly a scenario played out where Segue I passed through the Milky Way, which stripped it of most of its stars. Segue I came out the other side with high mass from its SMBH, but very few stars. This theory should be easy to confirm or deny by checking for the presence of a black hole by measuring the doppler effect or possibly by measuring the motion of stars at the galaxy’s center.

  5. RetardedFishFrog says

    Don Alexander?

  6. Jon Hanford says

    @RetardedFishFrog: The paper(arXiv:0809.2781v1) 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. You may want to check out the paper itself for further details on “individual stellar velocity measurement” that were made( including stars at the galaxy’s center, which included most of the stars measured). Both of these measurements did not correspond with a “dwarf supermassive black hole”, whatever that is. It looks like you’ll need to find another ad hoc explanation to replace the strong indication of dark matter in Segue 1.

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