Galactic collisions send stars shooting into space

TOM WILKIE

In the constellation of Fornax - the aptly named "furnace" - some 60 million light years above the southern skies, millions of stars are being torn from their rightful homes and expelled into the emptiness of intergalactic space as a result of the collision of galaxies.

Dr Tom Theuns, from the Department of Astrophysics at Oxford University, said that the stars were being tugged into the emptiness of space as a result of the tidal forces exerted by the gravitational pull of galaxies in collision.

Once released, the stars were free to wander within an area 3 million light years across. In comparison, our galaxy, the Milky Way, is a mere 60,000 light years across.

Fornax is a cluster of about 200 galaxies. Many of them are elliptical in shape rather than having the spiral form characteristic of the Milky Way. The elliptical structure is believed to be the result of the tidal forces from past collisions altering the pristine spiral shape of the galaxies, Dr Theuns said.

Computer simulations of the dynamics of colliding galaxies had predicted that, in addition to altering the galactic morphology, the collision should produce "debris" in the shape of lonely extra-galactic stars.

From the evidence of their shape, Dr Theuns believed that Fornax would be a good place to look for evidence of these lonely stars. However, the galaxy is so distant that it is impossible to observe individual stars even with the most powerful of telescopes. Instead he looked for patterns betraying the presence of "planetary nebulae" in the spectrum of the light coming from the cluster of galaxies. In our Milky Way there are about 10,000 planetary nuclei scattered among 100 billion stars; so Dr Theuns knewthat if he found just a few nebulae in any area, it would indicate there must be millions of other stars there too.

He used the European Southern Observatory's New Technology Telescope in the Chilean Andes to search for isolated planetary nebulae, and found "a couple of candidates".