Black hole jets stop stars forming

Galaxies aren't as massive as computer simulations suggested they should be - and now scientists have one explanation for this...

Past computer simulations have given broad predictions for galactic masses, but these have been much bigger than the actual masses deduced from telescope observations. This discrepancy has long been investigated by astrophysicists.

About a decade ago, theorists came upon the idea that the black holes might have a role to play. The jets they emit were introduced as a means to expel gas out of galaxies, or to prevent gas from clumping together: jets can be 10 billion times more powerful than the radiation emitted by our sun.

Dr Kalliopi Dasyra at the National and Kapodistrian University of Athens, Greece, and colleagues have provided the best observational evidence that these jets do indeed stir up and heat the interstellar gas.

'We could see from the start that there was something exciting in the data,' said Dr Dasyra, whose work is backed by a European Commission Marie Curie grant. 'But the data were complex, revealing multiple gas components that moved with different velocities.'

Dr Dasyra and colleagues' study focused on a nearby galaxy known as IC5063. Like most other galaxies, IC5063 is believed to have at its centre a supermassive black hole, which emits two jets of plasma in opposite directions. But unlike most other galaxies, the jet passes through a disk with dense gas clouds.

Analysing archival data taken from the Very Large Telescope in Chile, the researchers managed to extract the velocity and the temperature of the interstellar gas at four points along the jet trail.

They found that these quantities were greater along the jet than elsewhere. Such volatile conditions could keep the region impacted by the jet from cooling fast enough for the gas to clump together and form stars, said Dr Dasyra.

'Concrete evidence'

'This result is very important because it provides concrete evidence that the jet is responsible for expelling a large quantity of gas that would have instead formed new stars,' said Dr Francesco Tombesi, an astrophysicist at NASA's Goddard Space Flight Center in Maryland, US, who was not involved with the work. 'It provides a strong confirmation of the idea that supermassive black holes can significantly influence the evolution of galaxies.'

Evidence of the jets' interaction with interstellar matter has previously been found. But the discovery of multiple winds along the jet provides the most 'definitive' evidence of this interaction. A large part of the inner star-forming disk experiences these winds, which are detected even at distances as large as 3 000 light years away from the black hole, Dr Dasyra said.

The area around a black hole jet has more velocity and higher temperatures than elsewhere, meaning gaseous material such as molecular hydrogen cannot cool down enough to form stars. Researchers found evidence for jet-matter interactions, as seen in the coloured regions above. Image courtesy of Dr Kalliopi Dasyra

'When everything came through, it was a beautiful puzzle that we had solved,' she added.

Dr Dasyra has not closed the book on the galactic mass mystery, however. To do so, she needs evidence that there are indeed fewer stars being born in the jet region than elsewhere.

Finding that evidence will be tricky, because the researchers do not yet know whether the jets act long enough upon the gas to influence the formation of new stars. Dr Dasyra believes the possibility of finding the evidence will attract a great many astrophysicists.

'Whoever succeeds in this project will contribute a lot to the field,' she said.

By Jon Cartwright