Australasian Science: Australia's authority on science since 1938

A Black Hole’s Afterburn

By Stephen Luntz

A Black Hole’s Afterburn

Rather than one smooth path, images taken from the Australia Telescope Compact Array reveal a sequence of bright dots along the jet known as PKS 0637-752. Although PKS 0637-752 has been known for some time, and studied in depth by the Chandra X-Ray Telescope, it has never been imaged with such clarity before.

“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 Dr Leith Godfrey of the Curtin University node of the International Centre for Radio Astronomy Research.

“One intriguing possibility is that the pattern we see in this cosmic jet is produced in the same way as the pattern in the exhaust from fighter jet engines,” Godfrey adds. “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.”

These patches, known as mots, are commonly seen in quasar jets but Godfrey says: “The interesting thing is that these are quite regularly spaced, and there are lots of them, making this a really striking example. We can use the observational clue as a constraint on the source dynamics.”

Mots appear in any supersonic jet. “Hot gas comes out of a nozzle in one direction, with different pressure to the surrounding air,” Godfrey says. “The jet wants to come into pressure balance with the air surrounding it. It expands until it reaches equilibrium, but because it is supersonic it overshoots that pressure and becomes underpressured.”

From here the pressure swings back and forth like a pendulum, either too high or too low. “The communication of pressure differences is slower than the velocity of the jet, so you get standing criss-crossed shockwaves along the jet,” Godfrey explains.

Astronomers speculate the intergalactic medium is standing in for air.

PKS 0637-752 is pointed somewhat in our direction, so each mot is not necessarily 100,000 light years across, but Godfrey confirms that their scale is similar to that of the galaxy.

The jets are thought to prevent star formation by keeping the region at temperatures too hot to allow stars to take shape.

The glowing trail of material extending out from a super­massive black hole resembles the afterburn of a fighter jet, Australian astronomers have noted. However, in this case the trail is two million light years long, the distance from here to the Andromeda Galaxy.