News Release

Diagnosis murder

Peer-Reviewed Publication

University of Nottingham

Black holes have long been beloved of science fiction writers for their destructive capabilities and peculiar ability to warp space time. Now a study led by researchers from The University of Nottingham reveals the awesome power of supermassive black holes — the ability to strip massive galaxies of the cool gases required to form new stars, leaving ageing red giants to splutter out of existence with no stars to replace them..

The study, led by Asa Bluck in the School of Physics and Astronomy, used images of unprecedented depth and resolution from the Hubble Space Telescope and the Chandra X-Ray Observatory to detect black holes in distant galaxies. Researchers looked for galaxies emitting high levels of radiation and x-rays — a classic signature of black holes devouring gas and dust through accretion, or attracting matter gravitationally.

As this matter swirls around the event horizon of a black hole it heats up and radiates energy — as an accretion disc. The study, which was funded by the Science and Technology Facilities Council and NASA and was a collaboration between researchers at The University of Nottingham and Imperial College London, gleaned some startling results. In supermassive black holes this radiation can reach huge proportions, emitting X-ray radiation in far greater quantities then is emitted by the rest of the objects in the galaxy combined — meaning that the black hole 'shines' far brighter than the entire galaxy it lies at the heart of. In fact, the amount of energy released is sufficient to strip the galaxy of gas at least 25 times over.

Results have also shown that the vast majority of the X-ray radiation present in the universe is produced in these accretion discs surrounding supermassive black holes, with a small proportion produced by all other objects, including galaxies and neutron stars.

The accretions discs surrounding supermassive black holes produce so much energy that they heat up the cold gases lying at the heart of massive galaxies. The accretion disc shines across all wavelengths — from radio waves to gamma waves. This speeds up the random motions of the gas, making it rise in temperature and pushing it away from the galactic centre, where it becomes less dense. Gas needs to be cold and dense to collapse under gravity to form new stars, this resulting hot, low-density material must cool down before gravity will take effect — a process which would take longer than the age of the universe to achieve.

Old stars are therefore left to die out with no new stars replacing them, leaving the galaxy to grow dark and die. And by pushing gas away from the galactic centre, the accretion disc starves the supermassive black hole of new material to devour, leading to its eventual demise.

"It's thought that black holes form inside their host galaxies and grow in proportion to them, forming an accretion disc which will eventually destroy the host. In this sense they can be described as viral in nature," said Asa Bluck, a PhD student at the University and a Fellow of the Royal Astronomical Society. "Massive galaxies are in the minority in our visible universe — about one in a thousand galaxies is thought to be massive, but it may be much less. And at least a third of these have supermassive black holes at their centre. That's why it's so interesting that this type of black hole produces most of the X-ray light in the universe. They are the minority but they dominate energy output."

Asa will present these results at the Royal Astronomical Society National Astronomy Meeting in Glasgow on Friday 16 April.

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Notes to editors: The University of Nottingham is ranked in the UK's Top 10 and the World's Top 100 universities by the Shanghai Jiao Tong (SJTU) and Times Higher (THE) World University Rankings.

More than 90 per cent of research at The University of Nottingham is of international quality, according to RAE 2008, with almost 60 per cent of all research defined as 'world-leading' or 'internationally excellent'. Research Fortnight analysis of RAE 2008 ranks the University 7th in the UK by research power. In 27 subject areas, the University features in the UK Top Ten, with 14 of those in the Top Five.

The University provides innovative and top quality teaching, undertakes world-changing research, and attracts talented staff and students from 150 nations. Described by The Times as Britain's "only truly global university", it has invested continuously in award-winning campuses in the United Kingdom, China and Malaysia. Twice since 2003 its research and teaching academics have won Nobel Prizes. The University has won the Queen's Award for Enterprise in both 2006 (International Trade) and 2007 (Innovation — School of Pharmacy), and was named 'Entrepreneurial University of the Year' at the Times Higher Education Awards 2008.

Nottingham was designated as a Science City in 2005 in recognition of its rich scientific heritage, industrial base and role as a leading research centre. Nottingham has since embarked on a wide range of business, property, knowledge transfer and educational initiatives (www.science-city.co.uk) in order to build on its growing reputation as an international centre of scientific excellence. The University of Nottingham is a partner in Nottingham: the Science City.

More information is available from Asa Bluck on +44 (0)115 846 8857, +44 (0)7708 822644, ppxab3@nottingham.ac.uk or Tara De Cozar, Internal Communications Manager, on +44 (0)115 846 8545, tara.decozar@nottingham.ac.uk


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