CHAMPAIGN, Ill. — NASA’s Hubble Space Telescope is seeing double, uncovering two very close pairs of quasars that existed 10 billion years ago. The objects are close together because astronomers believe they resided in a pair of merging galaxies.
Research led by University of Illinois Urbana-Champaign astronomy and National Center for Supercomputing Applications professor Yue Shen offers a new way to probe collisions among galaxies in the early universe and the pairing of their supermassive black holes. The new study is published in the journal Nature Astronomy.
Quasars are beacons of intense light from the centers of galaxies that can outshine their entire galaxies. Powered by supermassive black holes, they feed on infalling matter and unleash a torrent of radiation.
“We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar,” Shen said. “So finding these double quasars is like finding a needle in a haystack.”
Quasars are scattered across the universe and were most abundant 10 billion years ago. Astronomers are seeing them now because it took 10 billion years for their light to travel to Earth.
Finding double quasars now offers evidence that it is possible to form a pair of supermassive black holes that may eventually coalesce to produce gravitational waves – ripples in the fabric of space – that astronomers will be able to detect sometime in the future, the researchers said.
The team is using Hubble, the European Space Agency’s Gaia space observatory and the Sloan Digital Sky Survey, as well as several ground-based telescopes, to compile a census of quasar pairs in the early universe.
Of the four targets selected using the data from Gaia and the Slone survey, Hubble revealed that two were close pairs of quasars, the study reports.
“Quasars made a profound impact on galaxy formation in the universe,” said study co-author Nadia Zakamska of Johns Hopkins University in Baltimore. “Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolved together.”
Finding them was not easy, the researchers said. Hubble is the only telescope with vision sharp enough to peer back to the early universe and distinguish two close quasars that are so far away from Earth. However, Hubble’s sharp resolution alone is not good enough to find these dual light beacons.
Astronomers first needed to figure out where to point Hubble to study them. The team enlisted the Sloan survey to compile a list of existing quasars and Gaia database to identify quasars that show a slight jigglelike movement.
That jiggle could be evidence of random fluctuations of light as each member of the quasar pair varied in brightness. Quasars flicker in brightness on timescales of 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 hunted for double quasars closer to Earth using different techniques with ground-based telescopes. “The new technique can not only discover dual quasars much farther away, but it is much more efficient than the methods we’ve used before,” she said.
The team is confident of its result but says there is a slight chance that the Hubble snapshots captured double images of the same quasar, an illusion caused by gravitational lensing.
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 is improbable because Hubble did not detect any foreground galaxies near the two quasar pairs.
The team also obtained follow-up observations with the National Science Foundation NOIRLab’s Gemini telescopes. “Gemini’s spatially resolved spectroscopy can unambiguously reject interlopers due to chance superpositions from unassociated star-quasar systems, where the foreground star is coincidentally aligned with the background quasar,” said co-author Yu-Ching Chen, a graduate student at Illinois.
The Hubble Space Telescope is a project of international cooperation 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, D.C.