We are used to thinking of black holes as cosmic cannibals with the supermassive variety swallowing up gas, dust and stars. But new research with the Hubble Space Telescope suggests that these monsters lurking at the hearts of galaxies may also help create new stars.

Centaurus A imaged from La Silla, Chile. The filament studied is within the green box. Credit: ESO/Vandame

An international team of astronomers studied a relatively nearby galaxy called Centaurus A which contains just such a black hole. Their results showed that far from hindering starbirth, this black hole seemed to be helping stars to form.

Active black holes can produce jets of material stretching millions of light-years. As they force their way through galactic gas – a raw ingredient from which many stars are made – they compress and heat it, pushing it out of the way.

A team led by Dr Stanislav Shabala of the University of Tasmania, Dr Mark Crockett of the University of Oxford, and Dr Sugata Kaviraj of Imperial College, London, used Hubble’s Wide Field Camera 3 (WFC3) to examine the centre of Centaurus A. This galaxy, which is also known as NGC 5128, lies 13 million light-years away and displays a prominent band of dust across it. Closer inspection with X-ray and radio telescopes reveal jets extending up to a million light-years from a black hole at its centre.

The astronomers used WFC3 to examine the inner filament which is visibly bright and also produces ultraviolet and X-ray emission. This allowed them to plot how stars had formed within the filament over time with unprecendented accuracy.

The team discovered that the tip of the filament closest to the outflow contains young stars, the ages of which are similar to the time since the black hole’s outflow “switched on”. However, there are no young stars further along the filament. They report in the journal Monthly Notices of the Royal Astronomical Society that this is exactly what is expected from an outflow overrunning a cloud of gas sitting in its path.

Ionised gas in the filament glows green in this image by Hubble's WFC3. Credit NASA/ESA.

According to the research, the densest central parts of the cloud are compressed and collapse to form stars, while the gas on the outskirts is swept away from the tip of the filament, like leaves in the wind.

Dr Shabala said: “This enhancement of star formation by outflows would have been even more important in a younger universe, where dense clumps of gas were much more common. Our study highlights the need to consider the role of positive feedback from outflows in our current paradigm of galaxy formation. It adds an exciting new piece to a great puzzle – that of understanding how galaxies came to be the way they are today.”