Using a host of powerful telescopes, a team of European astronomers has discovered an extremely luminous quasar that existed as early as 770 million years after the Big Bang. At a redshift (z) of 7.085, this object is now the most distant known quasar. The previous record holder has a redshift of 6.4.The team led by Daniel Mortlock and Stephen Warren (Imperial College, London) published its findings in June 30th's issue of Nature. According to the paper, the quasar’s spectrum shows signatures of gases that fogged the early universe and offers scientists a unique opportunity to study the events that altered the composition of the early cosmos.
At the heart of the quasar, however, lies a conundrum. The black hole that powers it is a monster. With a mass of two billion Suns, it strains currently accepted theories on black-hole formation and evolution.
Quasars form when gases swirling around supermassive black holes in distant galaxies heat up and emit radiation. Quasars that are relatively nearby can be observed in optical light. But in the case of very distant quasars, like this one, the expanding universe stretches the visible light into infrared wavelengths.This quasar was discovered using infrared deep-sky surveys from ground-based telescopes: ESO’s Very Large Telescope in Chile, the United Kingdom Infrared Telescope in Hawaii, and the Liverpool Telescope. Its presence was confirmed using photometric data from the Sloan Digital Sky Survey and spectroscopic readings from the Gemini North telescope.
As you go back in time, galaxies and quasars become increasing rare and harder to detect. It took the European team five years to make this discovery. “We expected to find quasars at redshift 7 eventually,” says Warren, “We estimated the number of quasars by taking the decline in numbers that is seen between redshifts 3 and 6, and extrapolating to redshift 7.”
Quasar ULASJ1120+0641, as it has been named, is not, however, the most distant object known in the deep sky. A gamma-ray burst has been identified at a redshift of 8.2, and a galaxy at a redshift of 8.6.
A probe into the Early Universe
This quasar opens a window into an era when a process called reionization was clearing the early cosmos of dense neutral hydrogen gas.Specific signatures for neutral hydrogen have been studied in the spectra of other quasars. Scattered clouds of neutral hydrogen seen in the late reionization period form absorption lines called Lyman-alpha forests. Denser fog from earlier periods presents as dark Gunn-Peterson troughs.
The spectrum of this quasar shows both these absorption patterns, indicating active reionization in the galaxy’s immediate vicinity. The team discovered that an area of some 6 million light-years around the quasar was completely reionized and transparent to light.
“This quasar is a vital probe of the early Universe. It is a very rare object that will help us to understand how supermassive black holes grew a few hundred million years after the Big Bang,” said Warren.
The Conundrum within the Quasar
While the discovery offers unprecedented opportunities, it also presents a quandary in the form of the monstrous black hole that powers the quasar.
There are two dominant models of black-hole formation. A star with the mass of 10 to 100 Suns can go supernova to spawn these structures. A second theory suggests that accreting clumps of gases can directly undergo core collapse to generate supermassive black holes.
But both models fail to explain how such an early black hole reached the mass of two billion Suns. One possible explanation is a rapidly growing black hole that developed from the explosion of a star the size of 500,000 Suns. But current theories cannot account for a star that size in the early universe. The same is true for large collapsing gas cores.
"This gives astronomers a headache," says Mortlock. "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!"
Cosmologist Mitch Begelman (Universitiy of Colorado) voiced another concern. A voracious monster this size could speed up the process of reionization beyond expected rates, unless cocooned by a sheath of dust.
Black-hole theorist Priyamvada Natarajan (Yale University), who spearheaded the model on self-regulating black holes, says the discovery was exciting for the same reasons that it was challenging. She emphasizes the need to find more giant black holes in order to come up with better models in the future. “The finding of a population is key to theoretical understanding," she explains. "We can always come up with explanations for single rare objects, but to explain a population you need a robust mechanism.”