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Old star missing link in galaxy evolution
Posted: 4 March 2010

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A newly discovered star that may have been among the second generation of stars to form after the big bang may be the missing link in models of our Galaxy's evolution.

The star, located in the dwarf galaxy Sculptor some 290,000 light years away, has a similar chemical makeup to the Milky Way's oldest stars. Dwarf galaxies contain just a few billion stars whereas Milky Way sized galaxies host hundreds of billions of stars. Recent studies have questioned the relationship between dwarf galaxies and the Milky Way, but the new research supports the theory that the Milky Way went through a cannibalistic phase, gorging on dwarf galaxies to attain its mass.

“If you watched a time-lapse movie of our Galaxy, you would see a swarm of dwarf galaxies buzzing around it like bees around a beehive,” explains Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, and lead author of the Nature paper reporting the finding . “Over time, those galaxies smashed together and mingled their stars to make one large galaxy – the Milky Way.”

The newly discovered red giant star S1020549 dominates this artist's conception. The primitive star contains 6,000 times less heavy elements than our Sun, indicating that it formed very early in the Universe's history. Located in the dwarf galaxy Sculptor some 290,000 light-years away, the star's presence supports the theory that our Galaxy underwent a "cannibal" phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks. Image: David A. Aguilar (CfA)

If this "bottom-up" model of galaxy formation is true, then the same kinds of stars should be found in both dwarf galaxies and the Milky Way. Old stars in the Milky Way's halo tend to be metal poor, that is, 100,000 times less metal-rich than the Sun (metals are chemical elements heavier than hydrogen or helium). Until now, surveys had failed to uncover any extremely metal-poor stars in dwarf galaxies, but it turns out that those surveys may have been biased in such a way that the surveys simply missed the most metal poor stars, even though they were there all along.

"It was a question of finding the right kind of star, and doing that required some new techniques," says Josh Simon, an astronomer at the Observatories of the Carnegie Institution. "Using earlier techniques, it was very difficult to recognize exactly which stars were the key ones to study."

The most accurate way to measure the composition of a star is to take a spectrum of it. The light of the star is dispersed into different colours, or wavelengths, and in certain places the intensity of the light of the star dips sharply. These locations are called absorption lines and are caused by a particular element in the star's atmosphere. Previous surveys had used the strongest absorption lines, representing calcium, to make assumptions about the metallicity of individual stars, but the new method estimates the metal abundances of large numbers of stars at once, based on the fainter iron absorption lines.

"Because the physics to convert iron line strengths to iron content is better understood than the physics to convert calcium line strengths to calcium or iron content, my measurements were more accurate than the calcium measurements, particularly for metal-poor stars," explains team member Evan Kirby who developed the new method. Looking in the Sculptor dwarf galaxy the team highlighted S1020549 as a metal poor star, finding it to have a metal abundance 6,000 times lower than the Sun, and five times lower than any other star found so far in a dwarf galaxy.

"The technique of measuring iron from iron absorption lines was based on observations at the medium-resolution spectrograph DEIMOS at the Keck II telescope," says Kirby. "The follow-up high-resolution spectroscopy was necessary to measure the amount of elements other than iron. It was the details of these other elements that showed how closely S1020549 resembles stars of similar metallicity in the Milky Way halo." 

The team measured the abundances of magnesium, calcium and titanium to that of iron and found that the ratios resembled those of old Milky Way stars, lending the first observational support that these galactic stars originally formed in dwarf galaxies.

“The original idea that the halo of the Milky Way was formed by destroying a lot of dwarf galaxies does indeed appear to be correct,” says Simon.

Future observations of the stellar chemistries of faint, distant dwarf galaxy stars will be ideal for the next generation of telescopes, such as the proposed 24.5 metre Giant Magellan Telescope.

Hubble Reborn
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3D Universe
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