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Gravity lenses focus attention on galaxy formation

Astronomers ID 19 more examples

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A US and Netherlands-led team of astronomers using the Hubble Space Telescope have identified 19 new examples of gravitationally lensed galaxies.

Among the new examples are eight so-called Einstein Rings, a rare phenomenon where an image of a hidden galaxy is stretched in a complete circle around the lensing object. Only three examples of visible-light Einstein rings were previously known.

Gravitational lensing takes place when a sufficiently massive object lies between the Earth and a light source, like a distant galaxy. The gravity of the intermediate object is strong enough that it will bend light from the object it obscures around itself, so that it is visible from Earth.

This takes the form of an arc, or of multiple images of the otherwise hidden object around the lensing object. When the two objects are lined up exactly, an Einstein ring is formed.

Beautiful as these objects are to look at, the astronomers at the Sloan Lens Survey (SLACS) have not been probing the universe for these things just for the aesthetics of it all.

The researchers, led by Adam Bolton of the Harvard-Smithsonian Center for Astrophysics in the US, and Leon Koopmans of the Kapteyn Astronomical Institute in the Netherlands, study the arcs and rings to calculate very precisely the mass of the foreground galaxies.

This information can reveal how much of the galaxy's matter is normal matter and how much is dark matter.

"Being able to study these and other gravitational lenses as far back in time as several billion years allows us to see directly whether the distribution of invisible and visible mass changes with cosmic time," says Koopmans.

"With this information, we can test the commonly held idea that galaxies form from collision and mergers of smaller galaxies."

The initial findings of the survey will appear in the February 2006 issue of the Astrophysical Journal.

Check out the pictures here. ®

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