Artist's conception of how the new quasar would appear close up. The very hot extremely luminous quasar at the centre of the image is very bright at ultraviolet wavelengths, and light from the quasar is ionising the surrounding gas, causing the red colour, which is the characteristic color of ionised hydrogen. In the background can be seen faint compact galaxies that have just been born; they contain hot stars that are also ionising their surroundings, but much less effectively than the quasar as they are far less luminous.
Credit: Gemini Observatory
Image of the new most distant quasar ULAS J1120+0641. The quasar is the red dot near the centre of the image. The picture is a colour composite made from images taken with the Liverpool Telescope and the United Kingdom Infrared Telescope. The quasar lies in the constellation Leo, a few degrees from the bright galaxy Messier 66.
Credit: The Liverpool Telescope and the United Kingdom Infrared Telescope
EDINBURGH: A newly discovered quasar is the most distant that has ever been seen, and has left researchers puzzled as to how the black hole at its centre could have grown so large, so fast.
This quasar, seen as it was around 12.9 billion years ago, could provide a vital probe for understanding the state of the early universe.
"This one of the very brightest quasars, and one of the earliest to form," said Australian astronomer Daniel Mortlock of Imperial College London, who led the research published this week in Nature. "It's part of the emerging story over the last 10 years of billion solar mass black holes in the early universe."
Clues to the early universe
Quasars are the shining beacons of the early universe, powered by matter falling onto supermassive black holes at the centres of young galaxies. Their intense brightness makes quasars visible at distances where most ordinary galaxies are too faint to be seen.
The quasar discovered by Mortlock and his colleagues is the most distant yet seen, at an estimated distance of 12.9 billion light years. That means we are seeing it as it was 12.9 billion years ago, when the universe was a mere 770 million years old.
The light from distant objects is stretched by the expansion of the universe, shifting it towards the red end of the spectrum. The more distant the object, the greater the redshift.
This was the key to discovering the new quasar, designated ULAS J1120+0641. Its redshift is so great that its light shines brightly in infrared, but is cut off sharply at a wavelength of 1 micrometer because hydrogen gas between us and the quasar absorbs light below this wavelength.
Panning for quasars
Crucially, the Sloan Digital Sky Survey (SDSS), an astronomical survey in visible light, only sees wavelengths below 1 micrometer, making this object invisible to it. However, in the United Kingdom Infrared Telescope (UKIRT) sky survey it shines out clearly.
Mortlock and his colleagues sought for distant quasars by identifying bright points of light in the UKIRT survey that did not appear in SDSS.
The search took five years, with many false positives - "like panning for gold," as Mortlock put it, "a lot of the time you see something shining in the pan that turns out to be a nail" - but eventually they found their quasar.
Rapid growth of supermassive black holes
It is astonishingly bright, over 60 trillion times brighter than the Sun, and the black hole at its centre is two billion times the Sun's mass. That's a puzzle for astronomers. Current theories say that supermassive black holes grow exponentially, doubling in mass every 50 million years as they drag in material from their surroundings.
Adding to the mystery
But at that rate, there was not enough time from when the universe formed for a black hole to grow so large. Explaining the masses of older quasars was already proving difficult. "There are a few plausible mechanisms, but none are particularly compelling," said Mortlock, "and this discovery makes the problem more acute."
Martin Hendry, of the University of Glasgow, agreed. "This is an exciting discovery. If ULAS J1120+0641 is confirmed to host a 2 billion solar mass black hole that will pose some serious challenges for our models of how seed black holes form and merge at high redshift."
Now that this quasar has been discovered, astronomers will study it in detail to probe the conditions of the early universe. Upcoming infrared surveys are expected to find quasars at even greater distances, and thus at even earlier times after the Big Bang.
But whether that resolves the mystery of how such massive black holes form, or intensifies it, remains to be seen.
