Quasars are precocious galaxies whose massive black holes began accreting matter, lighting them up, when the Universe was less than 800 million years old

Dr Xiaohui Fan, Institute for Advanced Study

Dr Donald Schneider of Pennsylvania State University says that two quasars have been seen with redshifts of 6.0 and 6.2.

They break the previous distance record established by last year's SDSS discovery of a quasar at redshift 5.8.

Redshift is a measure of distance used by astronomers. The light we see today from these newly discovered quasars would have set off 10 billion years ago.

Precocious galaxies

Because the Universe is expanding, the Doppler effect causes the light from distant objects to be "stretched" and shifted into the red end of the spectrum.

The SDSS's goal is the observation of 100,000 quasars

The greater the distance, the greater the redshift.

"Quasars are precocious galaxies whose massive black holes began accreting matter, lighting them up, when the Universe was less than 800 million years old," said Dr Xiaohui Fan, of the Institute for Advanced Study in Princeton, New Jersey, US, who led the team that identified the quasars.

The SDSS's goal is the observation of 100,000 quasars. To date, it has discovered 13,000, including the four most distant objects yet seen, and 26 of the 30 most distant quasars.

So far, so clear

Astronomers say that the key to SDSS's success in quasar spotting is its recording of precision digital data in five colours, which allows researchers to easily distinguish quasars from more common faint stars.

SDSS scientists follow up suspected high-redshift quasar candidates using larger telescopes to confirm their spectra. Astronomers captured the spectra of the two record breakers with a 3.5-metre (11.5 feet) telescope, located at the Apache Point Observatory in New Mexico, also the site of the SDSS telescope.

But it is not just the light from the distant quasar that carries information about the composition of the Universe. Because the intervening material between us and a distant quasar absorbs some of the quasar's light, that material, too, leaves its imprint in the quasar's spectrum that is observed by the SDSS.

Dr Donald York, of the University of Chicago, said: "In much the same way as an X-ray reveals the inside of the human body, this spectrum reveals a line of sight across the Universe, sometimes passing through hundreds of galaxies.

"With the SDSS spectrum of a single quasar, we can study hundreds of distant galaxies, some of which are just forming."

Previous record: A quasar with a redshift of 5.8