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‘Russian doll’ galaxy shows black holes’ true power

Artist's impression of a microquasar

Artist's impression of a microquasar, a black hole that produces huge jets of particles that 'pump' energy into gas clouds in surrounding space.

  • “Microquasar” within a galaxy is “powered” by a black hole
  • Shoots out jets of particles that emit radio waves
  • Jets pour energy into the clouds of gas that form stars

Following a study of what is in effect a miniature galaxy buried inside a normal-sized one—like a Russian doll—astronomers using the CSIRO’s Compact Array radio telescope have concluded that massive black holes are more powerful than we thought.

The study was made possible by a recent upgrade to the Compact Array, which can now do work of this kind five times faster than before.

The international team of astronomers, led by Dr Manfred Pakull at the University of Strasbourg in France, discovered a ‘microquasar’—a small black hole, weighing only as much as a star—that is shooting jets of radio wave-emitting particles (‘radio jets’) into the space surrounding it.

Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13 million light-years away in the Southern constellation Sculptor.

Earlier this year Pakull and colleagues studied S26 with optical and X-ray telescopes (the European Southern Observatory’s Very Large Telescope and NASA’s Chandra space telescope).


Some of the dishes of the Australia Telescope Compact Array.

Now they have made new observations with the Compact Array (near Narrabri, NSW). These show that S26 is a near-perfect mini-version of the much larger ‘radio galaxies’ and ‘radio quasars’.

Powerful radio galaxies and quasars are almost extinct today, but they dominated the early Universe, billions of years ago, like cosmic dinosaurs. They contain big black holes, billions of times more massive than the Sun, and shoot out huge radio jets that can stretch millions of light-years into space.

Escape from a black hole

We often hear that nothing can ever escape from a black hole, so how can these ones shoot out huge jets into space? The answer is that the material does not come from within the black hole itself, but from the region immediately surrounding it.

Because black holes have huge gravitational fields, they tend to attract or suck in lots of gas and interstellar dust. If this material passes the black hole’s ‘point of no return‘, called the event horizon, it will never come out again. But a lot of the material forms into flattened, swirling cloud—what astronomers call an ‘accretion disc’—that surrounds the black hole outside the event horizon.

In the process of falling in toward the black hole, this material gains energy and become very hot. Some of it is then shot out of the region surrounding the black hole, in directions perpendicular to the accretion disc. These are the jets.

Astronomers have been working for decades to understand the precise mechanisms by which the black holes form these giant jets, and how much energy those jets inject into the interstellar gas they travel through. That gas is the raw material for forming new stars, and the effects of the jets on star-formation have been hotly debated.

Composite image of S26 and NGC 7793

A composite image showing the position of the 'miniature galaxy' S26 within the galaxy NGC 7793. The inset of S26 is a radio image made with a CSIRO telescope; the 'hotspots' mark the ends of the jets shot out by the black hole (not visible in this picture). The main image of the galaxy is made from combined X-ray and optical data.

There is evidence that the jets help to get a galaxy’s star formation going, and there is counter evidence that jets can suppress the formation of stars. The question is far from settled, and much more work is needed to understand black hole jets.

Jets powered by black holes

“Measuring the power of black hole jets, and therefore their heating effect, is usually very difficult,” said co-author Roberto Soria (University College London), who carried out the radio observations.

“With this unusual object, a bonsai radio quasar in our own backyard, we have a unique opportunity to study the energetics of the jets.”

Using their combined optical, X-ray and radio data set of S26, the scientists were able to determine how much of the jet’s energy went into heating the gas around it, and how much went into making the jet itself visible at radio wavelengths.

They concluded that only about 1/1,000th of the energy went into creating the radio glow.

“This suggests that in bigger galaxies too the jets are about a thousand times more powerful than we’d estimate from their radio glow alone,” said Dr Tasso Tzioumis of CSIRO Astronomy and Space Science.

“That means that black hole jets can be both more powerful and more efficient than we thought, and that their heating effect on the galaxies they live in can be stronger.”

Adapted from information issued by CSIRO / Soria et al / CSIRO / ATCA; NGC 7793 – NASA, ESO and NOAO.

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