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| Superimposed mass density contours, caused by gravitational lensing of dark matter. Photograph taken with Hubble Space Telescope. Sister picture: image:bullet_cluster.jpg (Photo credit: Wikipedia) |
Gaze into the inky depths of the image above, detailing a portion of the night sky speckled with stars of varying brightness. Can you spot the ones that make up dwarf galaxy Segue 1? Probably not, since Segue 1 has emerged as the "darkest" galaxy yet known to astronomers -- not necessarily "dark" in terms of how much light that emits from it, but "dark" because it contains the highest concentration of dark matter found in a galaxy to date.
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Segue 1's heart of darkness first made news two years ago, when it was discovered by Yale astronomer Marla Geha and her Carnegie Institute of Science colleague, Joshua Simon, based on data gleaned from the Sloan Digital Sky Survey and the 10-meter Keck II telescope in Hawaii.
They found that a bunch of otherwise diverse stars -- as opposed to a cluster ripped out of a nearby galaxy -- were all moving together. This was unusual, and an indication that something was binding them together gravitationally. Geha and Simon hypothesized the culprit was dark matter.
Dark matter likely makes up around 23% of all matter in the universe. But scientists thus far have not been able to observe it directly, because it interacts so weakly with ordinary matter; we only infer its existence from detecting its gravitational fields.
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Fritz Zwicky first noticed this phenomenon in 1933 when he concluded that galaxies in the Coma cluster were moving so quickly that they should be able to escape from the cluster if visible mass was the only thing contributing to the cluster's gravitational pull. Since the cluster hadn't flown apart, he proposed the existence of "dark matter" to account for the observational data.
In the 1960s, Vera Rubin and Kent Ford confirmed Zwicky's theory when their spectral analysis revealed that the outer stars in selected spiral galaxies were orbiting just as quickly as those at the center. The visible matter wasn't sufficient to account for this; the spiral galaxy should be flying apart. Clearly, there had to be some kind of hidden "dark" mass adding to the galaxy's gravitational influence.
In fact, these so-called WIMPs (Weakly Interacting Massive Particles) do not interact with themselves in the same way that "normal" matter does. This is can be clearly seen in the now-famous "Bullet Cluster" image, in which the dark matter is separate from the hot cluster gas (right).
Segue 1 shows a similar composition: it seems to have 3400 times more mass than can be accounted for by the stars visible to astronomers, according to a new paper in the May 2011 issue of The Astrophysical Journal
. Geha and Simon have spent the last two years working with Keck II to study the motions of these stars further, analyzing not just how they move with respect to our own Milky Way galaxy, but also in relation to each other.
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Specifically, the stars should all be moving at about the same speed if their mass made up the bulk of Segue 1. That's not what Geha and Simon found. Instead, the stars are moving at varying rates, some very slow, some as fast as 224 km/sec.
The astronomers calculated how much mass would be needed to account for those varying speeds, and came up with a whopping 600,000 solar masses. There are only about 1000 stars in Segue 1, however, all close in size to our sun, so they only account for 1000 solar masses. Dark matter must account for the rest.
The next step is to search for further verification of the concentration of dark matter in Segue 1 by detecting the telltale gamma ray bursts that should arise from dark matter particle/antiparticle pairs colliding and annihilating. The Fermi Gamma Ray Telescope is the best available instrument for this at the moment, but it might not be sufficiently strong to detect these very faint glimmers of gamma ray bursts. At least, it hasn't found any yet.
There may even be other, even "darker" dwarf galaxies than little old Segue 1 lurking around the Milky Way, waiting to be discovered by adventurous astronomers eager to probe their hearts of darkness.
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