The Hubble Space Telescope of NASA is’seeing double.’ Hubble astronomers discovered a pair of quasars that are so close to each other that they appear to be a single object in ground-based telescopic photos, but not in Hubble’s crisp view. The quasars, according to the researchers, are very close to each other because they are located in the cores of two merging galaxies. The team then won the “daily double” by discovering yet another quasar pair in another colliding galaxy pair.
A quasar is a brilliant beacon of intense light that shines from the center of a distant galaxy and can outshine the entire galaxy. It is propelled by a supermassive black hole that devours inflating matter, releasing a torrent of radiation.
“We estimate that in the distant universe, there is one double quasar for every 1,000 quasars. Finding these double quasars is akin to looking for a needle in a haystack “Yue Shen of the University of Illinois at Urbana-Champaign was the lead researcher.
According to the researchers, the discovery of these four quasars provides a new way to investigate galaxies colliding and supermassive black holes merging in the early universe. Quasars are found all over the sky and were most common 10 billion years ago. There were many galaxy mergers back then, which fed the black holes. As a result, astronomers believe there should have been a lot of dual quasars around at the time.
NASA’s Hubble Space Telescope is “seeing double.” Peering back 10 billion years into the universe’s past, Hubble astronomers found a pair of quasars that are so close to each other they look like a single object in ground-based telescopic photos, but not in Hubble’s crisp view.
“This truly is the first sample of dual quasars at the peak epoch of galaxy formation that we can use to probe ideas about how supermassive black holes come together to eventually form a binary,” said Johns Hopkins University’s Nadia Zakamska.
The team’s findings were published in the online edition of Nature Astronomy on April 1st. Shen and Zakamska are part of a team that is compiling a comprehensive list of quasar pairs in the early universe using Hubble, the European Space Agency’s Gaia space observatory, the Sloan Digital Sky Survey, and several ground-based telescopes.
The findings are significant because the role of quasars in galactic encounters is critical to galaxy formation, according to the researchers. As two nearby galaxies begin to gravitationally distort each other, the material is funneled into their respective black holes, igniting their quasars.
Over time, the radiation from these high-intensity “light bulbs” creates powerful galactic winds that sweep away the majority of the gas from the merging galaxies. Star formation ceases when galaxies are deprived of gas, and the galaxies evolve into elliptical galaxies.
“Quasars make a profound impact on galaxy formation in the universe,” Zakamska said. “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 evolve together.”
So far, astronomers have discovered over 100 double quasars in merging galaxies. None of them, however, are as old as the two double quasars studied in this study. According to the Hubble images, the quasars in each pair are only about 10,000 light-years apart. Our Sun, by comparison, is 26,000 light-years away from the supermassive black hole at the center of our galaxy.
The host galaxies will eventually merge, followed by the quasars, resulting in an even more massive, single solitary black hole. It wasn’t easy to track them down. Hubble is the only telescope capable of peering back into the early universe and distinguishing two close quasars that are so far away from Earth. However, Hubble’s sharp resolution alone isn’t good enough to find these dual light beacons.
Astronomers had to first figure out where to point Hubble in order to study them. The difficulty is that the sky is a tapestry of ancient quasars that flared to life 10 billion years ago, only a tiny fraction of which are dual. To compile a group of potential candidates for Hubble to observe, it took an imaginative and innovative technique that required the assistance of the European Space Agency’s Gaia satellite and the ground-based Sloan Digital Sky Survey.
The Sloan telescope, located at Apache Point Observatory in New Mexico, creates three-dimensional maps of objects in the sky. The team combed through the Sloan survey to find quasars to investigate further.
The researchers then enlisted the assistance of the Gaia observatory to help them identify potential double-quasar candidates. Gaia precisely measures the positions, distances, and motions of nearby celestial objects. However, the team created a new, innovative application for Gaia that could be used to explore the far reaches of the universe. They searched the observatory’s database for quasars that resemble the apparent motion of nearby stars. In the Gaia data, the quasars appear as single objects. Gaia, on the other hand, can detect a subtle, unexpected “jiggle” in the apparent position of some of the quasars it observes.
The quasars aren’t moving through space in any discernible way, but their jiggle could be due to random fluctuations in light as each quasar pair varies in brightness. Quasars flicker in brightness over timescales ranging from days to months, depending on the feeding schedule of their black hole.
The quasar pair’s alternating brightness is analogous to seeing a railroad crossing signal from a distance. The sign appears to “jiggle” as the lights on both sides of the stationary signal alternately flash.
When Hubble observed the first four targets, its sharp vision revealed that two of the targets are two close pairs of quasars. It was a “light bulb moment” for the researchers, who said it validated their plan to use Sloan, Gaia, and Hubble to hunt for the ancient, elusive double powerhouses.
The Hubble confirmation was a “happy surprise,” according to team member Xin Liu of the University of Illinois at Urbana-Champaign. She has long been on the lookout for double quasars closer to Earth, employing various techniques with ground-based telescopes. “Not only can the new technique detect dual quasars much further away, but it is also much more efficient than previous methods,” she explained.
Their Nature Astronomy paper is a “proof of concept that really demonstrates that our targeted search for dual quasars is very efficient,” according to team member Hsiang-Chih Hwang, a graduate student at Johns Hopkins University and the Hubble program’s principal investigator. “It opens up a new avenue where we can collect a lot more interesting systems to follow up on, which astronomers were unable to do with previous techniques or datasets.”
The team also obtained follow-up observations using the Gemini telescopes at the National Science Foundation NOIRLab. “Gemini’s spatially-resolved spectroscopy can unambiguously reject interlopers from unassociated star-quasar systems, where the foreground star is coincidentally aligned with the background quasar,” said team member Yu-Ching Chen, a graduate student at the University of Illinois at Urbana-Champaign.
Although the team is confident in their findings, they acknowledge that there is a slight possibility that the Hubble snapshots captured two images of the same quasar, an illusion caused by gravitational lensing. When the gravity of a massive foreground galaxy splits and amplifies the light from a background quasar, it creates two mirror images. The researchers believe this scenario is unlikely because Hubble did not detect any foreground galaxies near the two quasar pairs.
Galactic mergers were more common billions of years ago, but a few still occur today. NGC 6240, a nearby system of merging galaxies with two, possibly three, supermassive black holes, is one example. Our Milky Way galaxy will collide with the neighboring Andromeda galaxy in a few billion years, resulting in an even closer galactic merger. The galactic conflict would most likely feed the supermassive black holes at the heart of each galaxy, igniting them as quasars.
Future telescopes may provide more information about these merging systems. NASA’s James Webb Space Telescope, an infrared observatory set to launch later this year, will investigate the host galaxies of the quasars. Webb will display galactic merger signatures such as starlight distribution and long streamers of gas pulled from interacting galaxies.
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