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Friday, November 5, 1999 Published at 14:13 GMT


Dance of the galaxies

Ships that pass in the night: But these galaxies are twinned for ever

By BBC News Online Science Editor Dr David Whitehouse

The Hubble Space Telescope has photographed two stately, spiral galaxies, passing close to each other like ships in the night.

The larger and more massive galaxy is called NGC 2207 (on the left in the image); the smaller one is called IC 2163.

Strong tidal forces from NGC 2207 have distorted the shape of IC 2163, flinging out stars and gas in long streamers into the vast wastes of intergalactic space, stretching a hundred thousand light-years toward the right-hand edge of the image.

Computer simulations of the event demonstrate the, by human standards, leisurely timescale over which such galactic collisions take place. The calculations suggest that IC 2163 is swinging past NGC 2207 in a counter-clockwise direction, having made its closest approach 40 million years ago.

Similar process

IC 2163, however, does not have enough speed to escape from the gravitational pull of NGC 2207. It is destined to be pulled back and swing past the larger galaxy again and again in the future.

[ image: A familiar pattern for other galaxies]
A familiar pattern for other galaxies
The star systems will orbit each other almost forever, forming a binary galaxy. But eventually, billions of years from now, they will merge into a single, more massive galaxy. It is believed that many present -day galaxies, including our own Milky Way, were formed from a similar process of coalescence of smaller galaxies occurring over billions of years.

In detail, the Hubble image reveals dust lanes in the spiral arms of NGC 2207, clearly silhouetted against IC 2163, which is in the background. A series of parallel dust filaments can be made out along the tidally stretched material on the right-hand side.

These large concentrations of gas and dust in both galaxies may well erupt into regions of active star formation in the near future.

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