An international team of astronomers announced today the discovery of the most distant known supermassive black hole, seen as a luminous quasar caused by gas falling into the black hole.
The discovery came to light using data from an ongoing infrared sky survey being conducted at the United Kingdom Infrared Telescope (UKIRT) and critical follow-up confirmation observations with the Gemini North telescope, both on Mauna Kea in Hawaii. The results are presented in the June 30, 2011 issue of the Journal Nature.
The light from the quasar started its journey toward us when the universe was only 6% of its present age, a mere 770 million years after the Big Bang, at a ‘redshift’ of about 7.1.
“This gives astronomers a headache,” says lead author Daniel Mortlock, from Imperial College London. “It’s difficult to understand how a black hole a billion times more massive than the Sun can have grown so early in the history of the universe. It’s like rolling a snowball down the hill and suddenly you find that it’s 20 feet across!”
However, as well as being a headache, the new quasar is a great opportunity, because it allows scientists to measure the conditions in the gas that the quasar’s light passes through on its way to us.
“What is particularly important about this source is how bright it is,” says Mortlock. “It’s hundreds of times brighter than anything else yet discovered at such a great distance. This means that we can use it to tell us for the first time what conditions were like in the early universe.”
Cosmologists are extremely keen to measure the state of gas in the early universe, to understand the process of how stars and galaxies formed. Most of the gas in the universe is hydrogen, and most of it is ionized at the present time, meaning that the electrons have been stripped off the protons. As one looks further away and thus further back in time, one should eventually reach the time when the gas was neutral, with the electrons and protons combined as atoms, before most of the stars in the universe have formed, over 12 billion years ago.
The transition between these periods is the epoch of reionization, a milestone in cosmic history. The light from the new quasar displays the characteristic signature of neutral gas. This signature, showing the quasar is beyond the epoch of reionization, was predicted in 1998 but has never been observed before.
“Being able to analyze matter at this critical juncture in the history of the universe is something we’ve been long striving for but never quite achieved. Now it looks like we have crossed the barrier with this observation,” said Prof. Steve Warren, leader of the quasar team. “It’s like discovering a new continent which we can now explore.”
The quasar, named ULAS J1120+0641, was discovered in the UKIRT Infrared Deep Sky Survey (UKIDSS) a new map of the sky at infrared wavelengths. Such very distant, highly redshifted objects are much more easily found in infrared light.
“It was for just this sort of discovery that we began this ambitious survey in 2005,” said Prof. Gary Davis, Director of UKIRT.
To find the quasar the team sifted through images of over 10 million sources.
“We’d been searching for five years, and hadn’t found anything, and were beginning to lose heart,” said Warren. “It gave us a terrific jolt when we found it as we hadn’t really expected to discover anything quite so far away.”
To confirm that the object was really a distant quasar and measure its distance, in December 2010 the team made further observations with the 8-meter Gemini North telescope, UKIRT’s neighbor on Mauna Kea, using the Gemini Near-Infrared Spectrograph (GNIRS).
“The timing was perfect,” recalled Kathy Roth, an astronomer at Gemini Observatory, “as we got the observation request just days after the spectrograph had been made available for science use at Gemini North. Once the measurements were made it became immediately obvious they had found what they were looking for.”
The team then quickly collected an additional set of detailed observations, with telescopes at the European Southern Observatory (ESO), and in the Canary Islands. Collectively, these observations from many facilities allowed a detailed study of the properties of the quasar itself, and of the surrounding gas.
The team plans further detailed observations of ULAS J1120+0641, but also hope to find more such distant but bright quasars. “There may be 100 such objects spread around the whole sky,” says Mortlock, “but finding them amongst the billions of other objects in astronomical images is a serious challenge!”
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