Supersized black hole discovery forces universal rethink

Massive galactic objects, billions of times heavier than the sun, could be more common than was thought, say scientists

Our universe could be riddled with monster black holes, new research has suggested.

The revelation comes after a black hole with a mass of 17bn suns was found in a large, virtually isolated galaxy 200m light years away.

Black holes are referred to as “supermassive” if they have masses of millions or billions of times more than the sun. Supermassive black holes with masses of more than 10bn suns have previously been found at the heart of large galaxies located in dense clusters in the universe. But this is the first time astronomers have found such an object lurking at the centre of a large galaxy in a relatively empty area of the universe.

“We didn’t expect to see such a huge black hole in a small place,” said Professor Chung-Pei Ma, an author of the study from the University of California, Berkeley.

That, she added, opens up an intriguing possibility. With such galaxies more common than rich clusters, such supermassive black holes could be rife.

“What this is saying is that you don’t need these galaxy clusters to grow very massive black holes,” says Professor Poshak Gandhi of the University of Southampton, who was not involved in the study. “That throws a wrench in the works of our understanding of how these monster black holes form – it throws the field wide open.”

Sky survey image of the galaxy NGC 1600, and a Hubble Space Telescope closeup of the bright centre of the galaxy where the 17-billion-solar-mass black hole resides
A sky survey image of the massive galaxy NGC 1600, and a Hubble Space Telescope closeup of the bright centre of the galaxy where the 17-billion-solar-mass black hole - or binary black hole - resides. Photograph: ESA/Hubble image courtesy of STScI

Writing in the journal Nature, a team of scientists from the US and Germany describe how the discovery of a supermassive black hole at the centre of a galaxy known as NGC 1600 arose from a large study into massive galaxies in the local universe. Appropriately termed Massive, the study combines data from the Hubble Space Telescope, the Gemini Telescope in Hawaii and the McDonald Observatory in Texas, with the goal of enabling scientists to unpick the secrets of supermassive black hole formation and the relationship of these hefty objects to their galaxy.

By studying the movement of stars within NGC 1600, the astronomers deduced that at the core of the galaxy lies a monster black hole with a mass equal to 17bn suns. “Black holes, by definition, are black – they don’t give out light so it is hard to study them. But they have very strong gravity so they will make the stars very close to them whizz around much faster than in their absence,” said Ma. “So we needed to study stars at the centre very close to the black hole in order to measure its mass.”

In the process, the authors made a further discovery, a dearth of stars in the immediate vicinity of the supermassive black hole. “They seem to be scared to get close to the black hole,” said Ma.

That, she said, could have arisen when two galaxies merged to form NGC 1600. During the event, they argue, supermassive black holes at the centre of each galaxy could have moved closer together and begun to circle each other, forming what is known as a binary. The gravitational influence of this system could have destabilised the orbits of nearby stars and hurled them away from the centre of the galaxy like a slingshot. “Each time they eject a star [the black holes] lose a bit of energy and the binary becomes smaller,” said Dr Jens Thomas, an author of the paper from the Max Planck Institute for Extraterrestrial Physics. “At some point the two black holes are so close to each other that they merge.”

But Gandhi believes further work is needed to back up the suggestion of a binary system. “It is possible, but there is not direct evidence for it at this stage,” he said.

The mass of the supermassive black hole also gives clues to the galaxy’s past, said Ma, with its size suggesting that it grew by “feeding” off vast quantities of gas. That, she added, hints that the galaxy was once a quasar – a bright, energetic source of radiation. “What we think is the quasar has quietened down to become this sleeping giant,” she said.

While NGC 1600 is the first such large, isolated galaxy known to have such a hefty supermassive black hole at its centre, the team are now keen to find out whether these monster objects are littered across the universe. “The next thing we want to find out is if other galaxies in such relatively common environments also have such big black holes,” said Thomas.

Gandhi believes the new discovery reinforces the wonders of the universe. “Black holes are certainly the most mysterious objects in the cosmos,” he said.


Nicola Davis

The GuardianTramp

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