Washington: Scientists have found the answer to why the black holes go quiescent. Radiation from black holes can push away the gas they feed on, a new study has revealed.

Using Gemini North telescope in Hawaii, an international team, led by University of Sydney, has discovered that a black hole creates a "wind" which pushes its own "food" of dust and gas out of reach.

The astronomers studied the galaxy Markarian 231, which lies 600 million light years away.

According to them, this is the process that turned actively feeding black holes -- common in the early universe -- into the quiescent ones found in galaxies today.

"It looks like they've found the 'off switch' for black holes. We've long suspected that a negative feedback process like this must be at work, but these observations are the first clear evidence of outflows that can starve a black hole of fuel," lead astronomer Prof Joss Bland-Hawthorn said.

Markarian 231 is a "train wreck" resulting from the collision of two galaxies. At its centre is a black hole at least ten million times the mass of the Sun, which is sucking in gas and dust from its immediate surroundings.

The black hole in Markarian 231 was known to produce narrow jets but the Gemini observations have revealed a broad outflow extending in all directions for at least 8000 lightyears around the galaxy's core.

Gas is streaming away from the galaxy's centre at speeds of over 1000 kilometres a second -- fast enough to travel from Sydney to Perth in four seconds. The flow is sweeping away huge amounts of gas.

"The fireworks of new star formation and black hole feeding are coming to an end, most likely as a result of this outflow," team member Dr David Rupke of Rhodes College in the US said.