By Paul Rincon

Science reporter, BBC News



Galaxies have been seen colliding before, but this form of galactic violence is rarely witnessed by astronomers.

The jet could have a profound effect on any planets in the jet's path and could also trigger a burst of star formation.

The findings are to be published in the Astrophysical Journal.

They were obtained using Nasa's space-based Chandra X-ray Observatory, its Hubble Space Telescope, and Spitzer Space Telescope, as well as the Very Large Array (VLA) and Merlin radio telescopes on the ground.

It is like a black hole bully, punching the nose of a passing galaxy

Neil Tyson, Hayden Planetarium

Most, if not all, galaxies - including our own Milky Way - are thought to host supermassive black holes at their galactic centres. A handful of these galaxies eject powerful jets from the vicinities of their black holes, and are known as radio galaxies - because jets are very "visible" at radio wavelengths.

The larger of the two galaxies in 3C321 - dubbed the "death star galaxy" by the astronomers - has a jet emanating from the vicinity of the black hole at its centre. The unfortunate smaller galaxy has apparently swung into the jet's line of fire.

Destructive force

A bright spot in some images shows where the jet has slammed into the side of the companion galaxy, dissipating some of its energy. After striking it, the jet has become disrupted and deflected.

Jets can race out at close to the speed of light and can travel vast distances. The jet in 3C321 was about 1,000 light-years across and might have travelled one or two million light-years from its origin.

The jet hits the galaxy at its edge

Enlarge Image



"This jet could be causing all sorts of problems for the smaller galaxy it is pummelling," said Dan Evans, lead author from the Harvard-Smithsonian Center for Astrophysics in Cambridge, US.

This type of event is rarely seen. But astronomers Steve Croft and Wil van Breugel have observed a gas-rich jet setting off star birth in an arc of stars known as Minkowski's Object.

The combined effects of radiation and particles travelling at almost the speed of light could have disastrous consequences for the atmospheres of any Earth-like planets lying in the path of the jet.

Although we call it a death star galaxy, it might eventually be a source of new life in the more distant galaxy

Martin Hardcastle, University of Hertfordshire

Neil Tyson, director of the Hayden Planetarium in New York, commented: "Black holes are famous for wreaking havoc on their environment. This particular black hole is disrupting its local region by dining on matter that wanders too close - which is the source of the energy for this jet.

"It also fires a jet out of the galaxy. So it is like a black hole bully, punching the nose of a passing galaxy."

Basic properties

"There are still basic unanswered questions about how these jets work," said co-author Martin Hardcastle of the University of Hertfordshire, UK.

"We don't know how exactly they're generated close to the black hole, what they're made of, how fast they're going, or how they evolve with time. So an object like 3C321 can act as an experiment which can give us an insight into the inner working of the jet."

The effect of the jet on the companion galaxy is likely to be substantial, because the galaxies in 3C321 are extremely close to one another. At only about 20,000 light-years apart, these galaxies lie approximately the same distance as the Earth is from the centre of the Milky Way.

It is possible that it would not all be bad news for the galaxy struck by the jet. The massive influx of energy and radiation from the jet may induce the formation of large numbers of stars and planets once its initial wake of destruction is complete.

"Although we call it a death star galaxy, in the end it might be a source of new life in the more distant galaxy," said Dr Hardcastle.

The images from the VLA and Chandra indicate that the jet started hitting the smaller galaxy just one million years prior to the light from the event arriving at Earth. This is a blink of the eye in the lifetime of 3C321, which marks it out as an important opportunity to study a rare astronomical phenomenon, say the astronomers.

Paul.Rincon-INTERNET@bbc.co.uk