Galactic afterburner offers cosmic clues

By Joanna Egan 23 October 2012
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A supersonic jet blasting from the centre of a distant galaxy has curious similarities to the afterburner of a jet engine.

A NEW IMAGE of matter blasting through the universe is providing scientists with clues to the inner workings of astronomical jets, and may shed light on how black holes affect the galaxies they sit within.

The image shows a jet of plasma – superheated gas – travelling more than two million light years after being shot out from a supermassive black hole at the centre of a distant galaxy. Remarkably, the jet (called ‘PKS 0637-752’) bears a striking resemblance to the afterburner exhaust of fighter jets on Earth.

Research published in the Astrophysical Journal Letters reveals how patterns in the astronomical jet’s emissions, seen in the image, mirror those observed in the outflow of fighter jet engines.

Black hole plasma jet

Dr Leith Godfrey, from the Curtin University node of The International Centre for Radio Astronomy Research, and his team created the image using the Australia Telescope Compact Array (ATCA), in Narrabri, NSW. This is a series of six radio antennas spread over 6km, which capture radio waves from the sky.

“We made an image of the galaxy with the radio telescope using new higher frequency receivers,” Leith told Australian Geographic. “This enabled us to see the jet with more detail than had previously been possible. What was immediately striking to us was the regular pattern of alternating bright and dark regions along the jet, which had not been seen before.”

The jet’s series of bright patches resembles ‘shock-diamonds’ that occur in supersonic jets, such as the flow from fighter jet engines, here on Earth. A series of bright, diamond-shaped shock waves occurs when a fighter jet’s exhaust gases differ in pressure from the ambient air pressure.

Similarity to aeroplane jet engines

“If the brighter patches are caused by the same process in astronomical jets as they are in earthly jet engines, then the distance between them can give us important information about the power of the jet and the density of the surrounding space,” Leith says. “If we want to understand how galaxies form and grow, we need to understand these jets.”

Astronomical jets are streams of very hot gas that shoot through space, like water from a high pressure hose. They are produced when matter, such as material from nearby stars, falls onto a black hole, and they can reach up to 100 times the size of the Milky Way. This makes them the largest known objects in the universe. They are thought to heat up a galaxy’s atmosphere, preventing the formation of stars and limiting the galaxy’s growth.

Poorly understood cosmic phenomena

“Massive jets like this one have been studied for decades, since the beginning of radio astronomy, but we still don’t understand exactly how they are produced or what they’re made of,” says Leith.

Fred Watson, astronomer-in-charge of the Australian Astronomical Observatory, agrees. “Jets of this kind are among the most poorly-understood cosmic phenomena,” he says, commenting that our understanding of the universe is evolving with improved observing facilities.

“This very nice piece of research shows how intriguing new science can come from revisiting previously-studied objects with new equipment,” he says. “If the interpretation that we are seeing shock waves in the jet is correct, it has enormous implications for our understanding of the evolution of galaxies. It could give us new insight into the mechanisms that quench star-formation in young galaxies.”