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Most distant quasar
shines brightly

Posted: 01 July 2011

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A team of European astronomers, including UK astronomers, have discovered a bright quasar that has been beaming light since the Universe was a mere 770 million years old.

The brilliant beacon, named ULAS J1120+0641, is powered by a black hole with a mass two billion times that of the Sun. Located at a redshift – a term relating to astronomical distances – of 7.1, its light has taken 12.9 billion years to reach us. The next most distant quasar is seen at 870 million years after the big bang, or a redshift of 6.4, although gamma ray bursts have been detected at greater distances of 8.6 and 8.2 redshifts.

Artist's impression of distant quasar ULAS J1120+0641. Image: ESO/M. Kornmesser.

“We think there are only about 100 bright quasars with redshift higher than 7 over the whole sky,” says Daniel Mortlock of Imperial College London, and lead author of the paper that appears in the 30 June edition of the journal Nature. “Finding this object required a painstaking search, but it was worth the effort to be able to unravel some of the mysteries of the early Universe.”

The quasar was initially spotted using WFCAM, an infrared camera on the UK Infrared Telescope in Hawaii, and confirmed by observations made with the Liverpool Telescope, Gemini North telescope and the European Southern Observatory’s Very Large Telescope. Over 10 million sources were analysed before the quasar was discovered, but finding such a high mass black hole is difficult to explain so early in the Universe.

"The simplest models of black hole formation just can't create a two billion solar mass black hole so soon after the big bang, so its existence is something of a problem for theoretical physicists," Mortlock tells Astronomy Now, who adds that explanations for its existence require the first stars to be extremely massive or require black holes merging much more often than generally believed. "Given how hard it was to find even one bright quasar this early in the Universe's history, there are no immediate prospects for making the sort of "population census" that might reveal more about their formation mechanisms, so I think this will remain an unanswered question for some time."

The key insight the quasar has provided so far is into conditions in the early Universe, specifically to a time period known as the reionization epoch which persisted from around 150 to 800 million years after the big bang, when intense ultraviolet radiation from the first stars began breaking apart the neutral hydrogen gas that permeated the early Universe.

"The way the light from the quasar is absorbed by the hydrogen gas immediately in front of it implies that it was maybe 10 or 50 percent neutral at that time – whereas even "just" 100 million years later it was only 0.1 percent neutral," explains Mortlock. "These inferences will become more coherent as we find more such objects, but the key thing about this quasar is that it is the first bright source we've found that we're seeing in a fairly un-ionized (i.e. neutral) Universe."

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