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CHAMPAIGN, IL – NASA’s Hubble Space Telescope sees itself doubly, revealing two very close pairs of quasars that existed 10 billion years ago. The objects are close together because astronomers believe they were in a pair of merging galaxies.

The research, led by the University of Illinois Urbana-Champaign Astronomy and the National Center for Supercomputing Applications, Professor Yue Shen, offers a new way to study collisions between galaxies in the early universe and the pairing of their supermassive black holes. The new study was published in the journal Nature Astronomy.

Quasars are beacons of intense light from the centers of galaxies that can outshine all of their galaxies. Powered by supermassive black holes, they feed on infallible matter and release a beam of radiation.

Yue Shen, professor of astronomy in Illinois

Photo by L. Brian Stauffer


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“We estimate that in the distant universe there is one double quasar for every 1,000 quasars,” Shen said. “So finding these double quasars is like finding a needle in a haystack.”

Quasars are scattered throughout the universe and were most common 10 billion years ago. Astronomers are seeing them now because it took 10 billion years for their light to reach Earth. Finding double quasars is now providing evidence that it is possible to form a pair of supermassive black holes that can eventually grow together and create gravitational waves – waves in the fabric of space – that astronomers will be able to detect at some point in the future, the researchers said.

The team uses Hubble, the European Space Agency’s Gaia Space Observatory, and the Sloan Digital Sky Survey, as well as several ground-based telescopes, to take a count of the quasar pairs in the early universe.

Of the four targets selected using data from Gaia and the Slone survey, Hubble found that two were close pairs of quasars, the study reports said.

“Quasars have profoundly influenced the formation of galaxies in the universe,” said study co-author Nadia Zakamska of Johns Hopkins University in Baltimore. “Finding double quasars in this early epoch is important as we can now test our longstanding ideas of how black holes and their host galaxies evolved together.”

Finding them wasn’t easy, said the researchers. Hubble is the only telescope with a vision sharp enough to return to the early universe and distinguish two nearby quasars that are so far from Earth. Hubble’s sharp resolution alone is not enough, however, to find these dual light beacons.

The astronomers first had to figure out where to point Hubble to study them. The team commissioned the Sloan survey to compile a list of existing quasars and the Gaia database to identify quasars that exhibited slight wobbly movement.

This wobbling could be an indication of random light fluctuations, as each member of the quasar pair varied in brightness. Quasars flicker in brightness on timescales from days to months, depending on their black hole’s feeding schedule, the researchers said.

Illinois astronomy professor and co-author Xin Liu called the Hubble confirmation a “happy surprise.” She has long searched for double quasars closer to Earth, using various techniques with ground-based telescopes. “Not only can the new technique detect double quasars much further away, but it is also much more efficient than the methods we used before,” she said.

The team is convinced of its result, but says the chances are small that the Hubble snapshots captured double images of the same quasar, an illusion created by gravitational lenses. This phenomenon occurs when the gravity of a massive foreground galaxy splits and amplifies the light from the background quasar into two mirror images. However, the researchers said this scenario was unlikely because Hubble did not detect any foreground galaxies near the two pairs of quasars.

The team also received follow-up observations with the National Science Foundation NOIRLab’s Gemini telescopes. “Gemini’s spatially resolved spectroscopy can clearly reject interlopers due to random overlays of unassociated star quasar systems where the foreground star is randomly aligned with the background quasar,” said co-author Yu-Ching Chen, a PhD student in Illinois.

The Hubble Space Telescope is an international cooperation project between NASA and the European Space Agency. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, DC

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