Findings: Heavenly halo circles stars

January 10, 2003

A vast doughnut of stars has been found surrounding the Milky Way galaxy. The ring had previously gone unnoticed because of its low density and the clutter of intervening stars and dust, writes Steve Farrar.

But analysis of new survey data has revealed glimpses of the vast structure that sits around the galaxy like a halo.

Nial Tanvir, lecturer in astrophysics at Hertfordshire University, and colleagues at Cambridge, Groningen, Strasbourg and Sydney universities, used the wide-field camera on the 2.5m Isaac Newton Telescope on La Palma to study and pinpoint the positions of large numbers of faint stars.

This led them to the unexpected discovery of a small section of the ring.

The team then compared their findings with data from the US Sloan survey, which also found evidence of stars beyond the disk of the Milky Way galaxy.

Further hints have subsequently emerged from other survey projects.

The ring seems to be made out of old stars. It encircles the galaxy at a distance of 30,000 light years from the Sun.

While the disc of the spiral galaxy is relatively flat, the ring is much broader.

Dr Tanvir said it was plausible that it had been formed from the remains of a satellite galaxy that had been pulled apart by the gravitational tide of the Milky Way.

"In this sense, it was a little like the rings of debris that surround some of the planets of the solar system, most notably Saturn.

Dr Tanvir said: "These are very large structures but are spread through a great amount of space and so are hard to detect."

Colleague Rodrigo Ibata, of the Observatoire de Strasbourg, added: "Until now we haven't been able to see the wood for the trees."

The discovery was announced at the American Astronomical Society's meeting this week.

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