Peering far across space and time, astronomers have located a luminous beacon aglow when the universe was still in its infancy. That beacon, a bright astrophysical object known as a quasar, shines with the luminosity of 63 trillion suns as gas falling into a supermassive black holes compresses, heats up and radiates brightly. It is farther from Earth than any other known quasar—so distant that its light, emitted 13 billion years ago, is only now reaching Earth. Because of its extreme luminosity and record-setting distance, the quasar offers a unique opportunity to study the conditions of the universe as it underwent an important transition early in cosmic history.
By the time the universe was one billion years old, the once-neutral hydrogen gas atoms in between galaxies had been almost completely stripped of their electrons (ionized) by the glow of the first massive stars. But the full timeline of that process, known as re-ionization because it separated protons and electrons, as they had been in the first 380,000 years post–big bang, is somewhat uncertain. Quasars, with their tremendous intrinsic brightness, should make for excellent markers of the re-ionization process, acting as flashlights to illuminate the intergalactic medium. But quasar hunters working with optical telescopes had only been able to see back as far as 870 million years after the big bang, when the intergalactic medium’s transition from neutral to ionized was almost complete. (The universe is now 13.75 billion years old.) Beyond that point, a quasar’s light has been so stretched, or redshifted, by cosmic expansion that it no longer falls in the visible portion of the electromagnetic spectrum but rather in the longer-wavelength infrared.
Daniel Mortlock, an astrophysicist at Imperial College London, and his colleagues used that fact to their advantage. The researchers looked for objects that showed up in a large-area infrared sky survey but not in a visible-light survey covering the same area of sky, essentially isolating the high-redshift objects. They could thus discover a quasar, known as ULAS J1120+0641, at redshift 7.085, corresponding to a time just 770 million years after the big bang. That places the newfound quasar about 100 million years earlier in cosmic history than the previous record holder, which was at redshift 6.44. Mortlock and his colleagues report their finding in the June 30 issue of Nature. (Scientific American is part of Nature Publishing Group.)
The ancient quasar was spotted in the Infrared Deep Sky Survey at the U.K. Infrared Telescope, or UKIDSS, an ongoing seven-year project. The light from ULAS J1120+0641 shows a much greater imprint from neutral intergalactic hydrogen than its nearer, lower-redshift counterparts. “What this object tells us is that at least in front of this quasar, along this line of sight, back at that epoch the universe was about 10 percent, and maybe 50 percent, neutral hydrogen,” Mortlock says. With more observations of ULAS J1120+0641, and perhaps the future discovery of more quasars at a comparable distance, astronomers and cosmologists will be better able to uncover the re-ionization history of the universe. “One of the reasons to look at these time-machine objects is to find out what was happening at that time,” he says.
To glow so brightly at that early epoch in cosmic history, the newfound quasar would have to be powered by a black hole roughly two billion times as massive as the sun, or 500 times the mass of the black hole at the center of our galaxy. But such heft requires an explanation. “The quasar itself is a remarkable object in that no one really knows how to form a black hole that massive, two billion solar masses, in what in cosmological terms is a relatively short time,” Mortlock says. In other words, the astrophysicists have found the cosmic equivalent of a newborn baby with the stature of a full-grown adult. “It’s essentially the hardest object to make in the early universe that we know about,” Mortlock adds. The gargantuan black hole’s existence, discovered through exhaustive telescopic observations, now becomes a challenge for theorists to address. “Assuming that the universe makes sense,” Mortlock says, “it has to form somehow.”
