Black holes giving life as well as death

Black holes had been thought to hinder the birth of stars, since their gravitational forces propel a strong flow of material that sweeps away the gas clouds from which stars are made. Now the scientists have found evidence that these same flows can also compress regions of gas and form them into new stars.

Black holes giving life as well as death
An image of the 'inner filament' of the galaxy Centaurus A taken with the Wide Field Camera 3 on the Hubble Space Telescope. The filament of ionized gas, seen here in green, begins near the newly formed stars on the right of the image, depicted here as blue-white dots. The unseen black hole is to the right of the image. The left to right distance across the image is three thousand light years. (Credit: Mark Crockett)

The team, led by Dr Sugata Kaviraj of Imperial College London and colleagues at the Universities of Tasmania and Oxford, have published their results in the February edition of the journal Monthly Notices of the Royal Astronomical Society.

Dr Kaviraj, from Imperial’s Department of Physics, said: “Black holes may have a significantly more complicated effect on the universe than scientists previously thought they did. We knew that the outflows produced by black holes could remove gas and quench star formation but few imagined that they could somehow help the birth of completely new stars.”

The centre of just about every galaxy is thought to host a black hole, a highly dense point in space with a strong gravitational pull. The black holes ‘switch on’ from time to time, driving material around them into outflows that can stretch for millions of light years. This flow ploughs through galactic gas, compressing, heating and pushing it out of the way. Much of this gas is the raw material from which stars are made, so the outflows significantly affect star formation in the galaxies that host them.

The astronomers used the Hubble Space Telescope’s Wide Field Camera 3 (WFC3) to study the central regions of Centaurus A, a bright galaxy 13 million light years away in the direction of the southern constellation of Centaurus. In visible light, a prominent belt of dust can be seen running across the galaxy and when observed at X-ray and radio wavelengths it has jets extending up to 1 million light years from a central black hole.

With WFC3, the scientists took a close look at the ‘inner filament’, a region located close to the outflow that is a source of ultraviolet light and X-rays, as well as being bright in visible light. Using the Hubble images, the team were then able to map out the star formation history of the filament with unprecedented accuracy.

They found that the tip of the filament closest to the outflow contains young stars, the ages of which are similar to the time since the outflow ‘switched on.’ However, there are no young stars further up the filament, as would be expected from an outflow overrunning a cloud of gas sitting in its path. The researchers say that instead, the densest central parts of the cloud are being compressed and these collapse to form stars, while the gas on the outskirts is swept away from the tip of the filament, like a pile of autumn leaves in the wind.

Dr Kaviraj said: “The study suggests this phenomenon may have been really common in the early universe, when there were a lot more gas clouds, and be responsible for the birth of many billions of stars at that time. It could even still be going on in galaxies today, although there are very few known instances of this process in our local neighbourhood. Now we might need to significantly revise the physics that describes black hole-galaxy interactions in our models of galaxy formation.”

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