The early universe was a bustling place where galaxies often collided with each other and even merged. Using NASA’s Hubble Space Telescope and other space-based and ground-based observatories, astronomers investigating these developments have made a rare and unexpected discovery: a pair of gravitationally bound quasars, both glowing inside two merging galaxies. They existed when the universe was only 3 billion years old.
Quasars are glowing objects powered by voracious, supermassive black holes that spew ferocious sources of energy as they fill up with gas, dust, and whatever else is within their gravitational reach.
“We don’t see many double quasars at this early time in the universe. And that’s why this discovery is so exciting,” 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 only just developed in the last 10 to 15 years. Today’s powerful new observatories have allowed astronomers to identify cases where two quasars are active at the same time and are close enough to merge.
There is increasing evidence that large galaxies form through mergers. Smaller systems come together to form larger systems and larger and larger structures. During that process, pairs of supermassive black holes should form inside the merging galaxies. “Knowing the parent population of black holes will eventually tell us about the appearance of supermassive black holes in the early universe and how frequent such mergers might be,” Chen said.
“We are beginning to reveal 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 actually tells us that this population exists, and we now have a method to identify double quasars that are separated by less than the size of a single galaxy.”
This was a search for a needle in a haystack that required the combined power of NASA’s Hubble Space Telescope and the WM Keck Observatories in Hawaii. Multi-wavelength observations from the Gemini International Observatory in Hawaii, NSF’s Karl G. Jansky Very Large Array in New Mexico, and NASA’s Chandra X-ray Observatory also contributed to understanding the dynamic duo. And, the ESA (European Space Agency) space observatory Gaia helped identify this double quasar in the first place.
“Hubble’s sensitivity and resolution provided images that allow us to rule out other possibilities for what we are seeing,” Chen said. Hubble shows unequivocally that this is a genuine pair of supermassive black holes, rather than two images of the same quasar created by close-up gravitational lensing. And Hubble shows a tidal feature of the merger of two galaxies, where gravity distorts the shape of the galaxies into two tails of stars.
However, Hubble’s sharp resolution alone is not good enough to search for these dual light beacons. The researchers enlisted Gaia, which launched in 2013, to identify potential double quasar candidates. Gaia measures the positions, distances and movements of nearby celestial objects with great precision. 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. Quasars appear as unique objects in the Gaia data because they are so close to each other. However, Gaia can pick up a subtle and unexpected “shake” that mimics an apparent change in position of some of the quasars it observes.
In reality, quasars do not move through space in any measurable way. Instead, its shaking could be evidence of random fluctuations in light as each member of the quasar pair varies in brightness on time scales of days to months, depending on its black hole’s feeding schedule. This alternating brightness between the pair of quasars is similar to seeing a railroad crossing signal from a distance. As the lights on either side of the stationary sign flash alternately, the sign gives the illusion of “motion.”
Another challenge is that because gravity warps space like a funhouse mirror, a foreground galaxy could split the image of a distant quasar in two, creating the illusion that it was actually a binary pair. The Keck telescope was used to make sure there is no lensing galaxy between us and the putative double quasar.
As Hubble looks into the distant past, this double quasar no longer exists. During the intervening 10 billion years, their host galaxies have likely settled into a giant elliptical galaxy, such as those seen in the local universe today. And quasars have merged to become one gigantic supermassive black hole at their 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 arose from one or more galaxy mergers in the last few billion years.
NASA’s upcoming Nancy Grace Roman Space Telescope, which has the same visual acuity as Hubble, is ideal for hunting for binary quasars. Hubble has been used to painstakingly take data on individual targets. But Roman’s wide-angle infrared view of the universe is 200 times larger than Hubble’s. “Many quasars could be binary systems. The Roman telescope can make great improvements in this research area,” Liu said.