News in Science

Stellar string caught orbiting black hole

Faraway wonder A necklace made up of thousands of stars has been seen for the first time circling around a monster black hole in a nearby galaxy.

The discovery, which has been accepted for publication in the Astrophysical Journal, will help scientists seeking to understand the complex relationship between galaxies and the black holes at their centres.

The stars were seen in four evenly-spaced clusters orbiting a supermassive black hole in the galaxy NGC-2110.

"These star clusters are incredibly bright, brighter than any known star cluster in our galaxy," says the study's lead author Mark Durre of Melbourne's Swinburne University.

"As far as I know this is the first time a string of four star clusters have been seen orbiting a supermassive black hole."

Into the heart of darkness

Durre, and his PhD supervisor Professor Jeremy Mould, made the discovery while using the ten-metre infrared Keck Telescope in Hawaii, to peer deep into the heart of the nearby galaxy.

"These star clusters hadn't been seen before because they're hidden by clouds of gas and dust around the black hole, but they can be detected in infrared radiation which penetrates the clouds," says Durre.

"They're brighter ... than the brightest star cluster in our galaxy called the Arches cluster."

The supermassive black hole at the centre of NGC-1210 is about a hundred times bigger than the one at the centre of the Milky Way, which has about 4.3 million times the mass of the Sun.

"It's quite common to have a star cluster around a supermassive black hole but that's usually just one, and it's usually more a collection of stars than a true cluster," says Durre.

Unlocking secrets

The discovery could help explain how black holes feed.

Gas from the stars fall into the black hole and heat up, with some of it ejected as jets of energy and matter, blowing any surrounding gas away from the black hole. This surrounding gas is used to make the next generation of stars.

Durre and Mould were trying to map the flow of gas around the black hole when they detected the stellar clusters.

"We saw the gas we were studying coming out from these clusters and flowing towards the black hole," says Durre.

"The big problem is, for gas to fall into the black hole, it first needs to lose angular momentum, otherwise it would just go screaming past without falling in. We didn't know how gas could lose enough angular momentum to fall in to the black hole."

"However if you've got these star clusters orbiting very close in to the black hole, then gas coming out of these clusters can flow into the black hole more successfully."