This artist's concept shows the brilliant glare of two quasars residing in the cores of two galaxies that are in the chaotic process of merging. The gravitational tug-of-war between the two galaxies ignites a firestorm of star birth. (Credit: NASA, ESA, Joseph Olmsted (STScI).)

Text adapted from NASA release originally published 4/5/2023.

The early Universe was a rambunctious place where galaxies often bumped into each other and even merged together. Astronomers perusing these developments were at the right time and the right place to come up with an unexpected and highly valued discovery—a pair of gravitationally bound quasars, both blazing away inside two merging galaxies. They existed when the Universe was just 3 billion years old.

Quasars are powered by voracious, supermassive black holes blasting out ferocious fountains of energy as they engorge themselves on gas, dust, and anything else within their gravitational grasp.

“We don't see a lot of double quasars at this early time. And that's why this discovery is so exciting. Knowing about the progenitor population of black holes will eventually tell us about the emergence of supermassive black holes in the early Universe, and how frequent those mergers could be,” said graduate student Yu-Ching Chen of the University of Illinois at Urbana-Champaign, lead author of this study.

Finding close binary quasars is a relatively new area of research that has just developed in the past 10 to 15 years. Today’s powerful new observatories have allowed astronomers to identifying instances where two quasars are active at the same time and are close enough that they will eventually merge.

There is increasing evidence that large galaxies are built up through mergers. Smaller systems come together to form bigger systems and ever larger structures. During that process there should be pairs of supermassive black holes formed within the merging galaxies. “We’re starting to unveil this tip of the iceberg of the early binary quasar population,” said Xin Liu of the University of Illinois at Urbana-Champaign. “This is the uniqueness of this study. It is actually telling us that this population exists, and now we have a method to identify double quasars that are separated by less than the size of a single galaxy.”

This was a needle-in-haystack search that required the combined power of NASA’s Hubble Space Telescope and the W.M. Keck Observatories. Multi-wavelength observations from the Gemini Telescope, the Very Large Array telescope, and NASA’s Chandra X-ray observatory also contributed to understanding the dynamic duo. And, ESA’s Gaia observatory helped identify this double quasar in the first place.

“Hubble’s sensitivity and resolution provided pictures that allow us to rule out other possibilities for what we are seeing,” said Chen. Hubble shows, unequivocally, that this is indeed a genuine pair of supermassive black holes, rather than two images of the same quasar created by a foreground gravitational lens. And, Hubble shows a tidal feature from the merging of two galaxies.

However, Hubble's sharp resolution alone isn't good enough to go looking for these dual light beacons. The researchers enlisted ESA’s Gaia observatory to pinpoint potential double-quasar candidates. Gaia measures the positions, distances, and motions of nearby celestial objects very precisely. But in a novel technique, it can be used to explore the distant Universe. Gaia’s huge database can be used to search for quasars that mimic the apparent motion of nearby stars. The quasars appear as single objects in the Gaia data because they are so close together. However, Gaia can pick up a subtle, unexpected "jiggle" that mimics an apparent change in position of some of the quasars it observes.

The Chandra X-ray observatory was also used to support the hypothesis of a pair of supermassive black holes. Bright supermassive black holes that are accreting material around them are expected to emit X-rays. The Chandra X-ray data was analyzed to search for the presence of two X-ray emitting sources, and evidence was found for extremely bright X-rays coincident with each resolved galaxy. The models used show that each X-ray source is quite luminous and would be hard to describe from physics other than accretion around two supermassive black holes. “X-rays are a great way to search for pairs of supermassive black holes, and it’s exciting that the Chandra observations support the findings of the other multi-wavelength observations,” said Adi Foord, a postdoctoral fellow at Stanford University who led the X-ray analysis. “Given how closely separated the galaxy merger is, using the Chandra X-ray observatory to search for X-ray emission from a pair of supermassive black holes was absolutely necessary,” adds Foord.

Another challenge is that because gravity warps space like a funhouse mirror, a foreground galaxy could split the image of a distant quasar into two, creating the illusion it was really a binary pair. The Keck telescope was used to make sure there's no lensing galaxy in between us and the suspected double quasar. Keck’s adaptive optics is sharper than Hubble, because it's a 10-meter-telescope as compared to Hubble’s 2.4-meter. Still, Hubble’s sensitivity was needed to see very faint background features near the binary quasar, such as gravitational tidal tails of stars that are forming in galaxy mergers. Those features can only be seen with space-based telescopes.

What makes this a rarity is that both quasars have to be simultaneously active at feasting on infalling gas. The chance that both quasars are active is small compared to the lifetime of the binary system. Statistically, for 100 supermassive black holes samples, there's only one that will be actively accreting at a given time.

Because Hubble peers into the distant past, these double quasars no longer exist. Over the intervening 10 billion years, their host galaxies have likely settled into a giant elliptical galaxy, like the ones seen in the local Universe today. And, the quasars have merged to become a gargantuan, supermassive black hole at its center. The nearby giant elliptical galaxy, M87, has a monstrous black hole weighing 6.5 billion times the mass of our Sun. Perhaps this black hole was grown from one or more galaxy mergers over the past billions of years.

The upcoming NASA Nancy Grace Roman Space Telescope, having the same visual acuity as Hubble, is ideal for binary quasar hunting. Hubble has been used to painstakingly take data for individual targets. But Roman’s very wide-angle view of the universe is 200 times larger than Hubble’s in infrared light. “A lot of quasars out there could be binary systems. The Roman telescope can do huge improvements in this research area,” said Liu.

The results were published in the April 5, 2023 issue of the journal Nature.

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