Posted: September 22, 2008
Segue 1 is one of about two dozen small galaxies orbiting our own Milky Way Galaxy. These objects are so faint and contain so few stars that at first astronomers thought they were tightly bound star clusters known as globular clusters. But by analysing the light coming from the objects, assistant professor of astronomy Marla Geha of Yale University, and colleagues, showed that these objects are actually galaxies. The astronomers expected the galaxies to have low masses to correspond with their low brightnesses, but were shocked to discover that they are between 100 and 1000 times more massive than supposed. Segue 1 topped the scale at a billion times dimmer than the Milky Way, but nearly a thousand times more massive than it appears.
"I'm excited about this object," says Geha. "Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass."
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. Image: Sloan Digital Sky Survey.
The astronomers estimated the mass of Segue 1 by measuring how fast the stars in the galaxy are moving, and found them to be moving more rapidly than expected, in line with a greater mass. This extra weight is attributed to the presence of dark matter dominating the galaxy.
“However, this conclusion rests on an assumption that these stars are moving under the influence of gravity from the mass of Segue 1 only,” says Geha. “While we don't have any evidence that Milky Way is affecting the motion of stars in Segue 1, we can't yet prove this is the case.”
Dark matter is thought to make up about 85 percent of the total mass in the Universe, but is invisible, and only inferred by its gravitational influence on other objects. 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, as well as the nature and distribution of dark matter in the Universe.
Current galaxy formation theories predict that dark matter comes in a wide variety of different sized clumps, with smaller clumps more prolific than more massive clumps. Our own Milky Way Galaxy resides in one of these more massive clumps and is expected to have several hundred smaller dark matter clumps surrounding it.
“Although theory can robustly predict the number of dark matter clumps around the Milky Way, it is much less able to predict how many of these will host stars,” Geha tells Astronomy Now. “Until a few years ago, the answer was very few and the idea was that there were many 'empty' dark matter clumps around the Milky Way. The recent discoveries of many new Milky Way satellite galaxies, including Segue 1, suggest that perhaps many of these dark matter clumps do indeed host stars, just not that many stars.”
In terms of galaxy formation, the discovery of Segue 1 and other ‘heavy’ faint galaxies suggests that making a small number of stars in dark matter clumps may be easier than previously thought. "The galaxies I now consider bright used to be the least luminous ones we knew about,” adds Geha. “It's a totally new regime. This is a story that's just unfolding."
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