Scientists searching for astronomical objects in the early universe, not long after the Big Bang, have made a record-breaking, two-for-one discovery.
Using ground-based telescopes, a team of astronomers have discovered the most distant supermassive black hole ever found. The black hole has a mass 800 million times greater than our sun, which earns it the “supermassive” classification reserved for giants like this. Astronomers can’t see the black hole, but they know it’s there because they can see something else: A flood of light around the black hole that can outshine an entire galaxy. This is called a quasar, and this particular quasar is the most distant one ever observed.
The light from the quasar took more than 13 billion years to reach Earth, showing us a picture of itself as it was when the universe was just 5 percent of its current age. Back then, the universe was “just” 690 million years old. The hot soup of particles that burst into existence during the Big Bang was cooling rapidly and expanding outward. The first stars were starting to turn on, and the first galaxies beginning to swirl into shape. Quasars from this time are incredibly faint compared to the nearest quasars, the light from some of which takes just 600 million light years to reach the Earth.
“It’s like finding the needle in a haystack,” said Eduardo Bañados, an astronomer at the Carnegie Institution for Science who led the international research team. Their double discovery is described in a study published Wednesday in Nature.
Black holes, mysterious as they are, are among the most recognizable astronomical phenomena in popular science. They’re pretty straightforward: Black holes are spots in space where the tug of gravity is so strong that not even light can escape. They gobble up gas and dust and anything that comes near, growing and growing in size. A supermassive black hole sits in the center of virtually all large galaxies, including the Milky Way. Astronomers can infer their existence by watching fast-moving stars hurtle around a seemingly empty, dark region.
Quasars, meanwhile, are a little trickier to understand, and you’d be forgiven for thinking they sound like something out of Star Trek. A quasar is, to put it simply, the product of a binge-eating black hole. A black hole consumes nearby gas and dust inside a galaxy with intense speed, and the violent feast generates a swirling disk of material around it as it feeds. The disk heats up to extreme temperatures on the order of 100,000 degrees Kelvin and glows brightly. The resulting light show is what we call a quasar, and what a light show it is.
“A quasar emits more light than an entire galaxy’s worth of stars, and it’s actually just a glowing disk of material that is the size of our solar system,” said Daniel Mortlock, an astrophysicist at Imperial College London and Stockholm University. In 2011, Mortlock and his colleagues reported their discovery of the most distant quasar found at the time.
The more material a black hole consumes, the bigger it becomes. Eventually, the black hole drains the surrounding area of material and has nothing to eat. The luminous disk around it shrinks and fades, and the quasar is extinguished. In this way, quasars—and the black holes that power them—are like volcanoes, erupting under one set of conditions and settling into dormancy under another.
Quasars were first detected in 1963 by the Dutch astronomer Maarten Schmidt with California’s Palomar Observatory. Astronomers thought these newly discovered points of light were stars because of their extreme brightness. But when they studied the spectrum of their light, they were stunned to find the “stars” were more than a billion light-years away. When light travels through space, it gets stretched thanks to the constant expansion of the universe. As it moves, it shifts toward redder, longer wavelengths. Astronomers can measure this “redshift” to figure out how long the light took to reach Earth, which indicates how far a certain object is. Schmidt and his fellow astronomers knew that for stars to appear so luminous to Earth from such great distances was impossible. They were dealing with completely new phenomena.
“They’re not something that anyone predicted at all,” Mortlock said. “Occasionally you get astronomical objects like [stars known as] brown dwarfs, where people had predicted that they would exist and waited for astronomy to find them. No one predicted anything like quasars. It’s one of those cases where our imaginations weren’t up to what nature turned out to provide.”
To find the latest record-breaking quasar, Bañados and his colleagues used computer algorithms to search through databases of large sky surveys. They selected points of light they suspected could turn out to be quasars and observed them with the telescopes at Las Campanas Observatory in Chile. One night in March of this year, they all gathered to look at the data, one quasar candidate at a time. Quasars, astronomers have found, are easily recognizable when raw data is plotted on a chart. The spectrum of a quasar—a plot of brightness against the wavelength of light—has a very distinctive shape. Features known as emission lines appear broad, rather than sharp, thanks to the Doppler effect, which means the object emitting the light it traveling at high speeds.
“These objects are so bright that basically in 10 minutes, I can know from the raw data if it’s a quasar or not,” Bañados said. They found a quasar in their search, and when they calculated its distance from Earth, they couldn’t believe what they’d found. The next day, Bañados started drafting proposals to get observation time on powerful telescopes around the world to further study this quasar.
From the data for the quasar, astronomers can infer the size of the black hole responsible for powering it. “To get a bright quasar like this, you have to build up a supermassive black hole,” Mortlock said.
Astronomers studied the galaxy where the black hole and its quasar reside using radio telescopes in the French Alps and New Mexico. They found that the galaxy, at a mere 690 million years, had “already formed an enormous amount of dust and heavy chemical elements. This means it must already have formed a large amount of stars.” Astronomers say they’ll need to rethink some existing models for the evolution of galaxies to explain how a young galaxy could accumulate so much matter so fast. The findings about the galaxy are published in a separate study in the Astrophysical Journal Letters.
Quasars are some of the best targets for studying the early universe. Like flashlights, they illuminate a cosmic time astronomers are still struggling to understand. The newly discovered quasar comes from a period in the universe’s history know as “the epoch of re-ionization,” when a mysterious source of radiation ionized hydrogen and transformed the gas in the universe from an indiscernible fog into something transparent. About this time, the first objects to radiate light also formed. The exact process, as well as which phenomenon happened first, remains poorly understood.
Mortlock said he feels some sense of ownership of the quasar he discovered, which is now the second-farthest ever spotted. To feel that way about an object billions of light-years away is “completely ridiculous,” he said with a laugh. “And it’s especially ridiculous because there was no way that the object we discovered was going to be the end of this process. As we get more data and observe larger areas of the sky and look more deeply, we’re always going to find more objects like this.”
Someday, Bañados’s discovery will be relegated to second place, too. “There must be more out there, especially fainter ones,” Bañados said. “I’m still searching for them.”
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