Extreme quasars illuminate the fast lives of the first black holes

Quasars are cosmic contradictions. They emit dazzling light, yet their brilliance stems from the darkest objects in space: supermassive black holes at the hearts of galaxies. Caught in the black hole’s great gravitational grip, gas whirls around and rubs against other gas, attaining tremendous temperatures and thereby lighting up intensely before plunging into the abyss.

The record-breaking quasars discovered recently have brought to light new questions about these extraordinarily bright beacons that existed chiefly in the ancient universe. In 2021, astronomers reported the most distant quasar ever seen and, in 2024, the most luminous quasar known. Image credit: NASA, European Space Agency, and J. Olmsted (Space Telescope Science Institute).

Recent discoveries of record-breaking quasars have pushed the boundaries of our understanding. In 2021, astronomers reported the most distant quasar ever seen and, in 2024, the most luminous quasar known. The most distant quasar shone less than a billion years after the universe’s birth, which means that its black hole grew up in a hurry. Did the black hole start small and rapidly swallow lots of new material, or did the black hole begin life big and grow even bigger? Meanwhile, the most luminous quasar demonstrates just how quickly a black hole can swallow gas and grow larger. Because supermassive black holes reside at the centers of most large galaxies, including our own, similar processes likely once affected the central regions of large galaxies throughout the cosmos.

Brilliant Discovery

California Institute of Technology astronomer Maarten Schmidt found the first quasar in 1963, when he realized that a blue “star” seemingly in our galaxy was actually far beyond it. The clue came from the blue object’s redshift. As light waves from a distant object speed through the fabric of space, space expands and gradually stretches them to longer, or redder, wavelengths. The farther the object, the more space the light has traveled through, and so the more stretched out the light waves become. Schmidt recognized that the object, named 3C 273, had spectral lines with wavelengths that were 16% longer than normal; in astronomical parlance, its redshift was 0.16. This redshift, coupled with the universe’s expansion rate, implies a distance of some 2 billion light-years. To be so easily visible across such a vast distance, the object must far outshine the entire Milky Way Galaxy (1).

The next year, American astrophysicist Hong-Yee Chiu called these objects quasars (2), a term that initially made a bigger hit with the public than with his peers. Only in 1970—well after the introduction of Quasar televisions—did a leading astronomical publication allow the word: “The Astrophysical Journal has up till now not recognized the term ‘quasar’; and it regrets that it must now concede,” the editor lamented (3).

“Quasar” stands in part for “quasi-stellar object”: Like stars, quasars appear to us as mere points of light. This is because the hot, luminous gas swirling around the supermassive black hole occupies a region smaller than our solar system—minuscule in cosmic terms—that lies extremely far from Earth and outshines the host galaxy.

Quasars were most common long ago, between redshifts 2 and 3, when the universe was 2.1 billion to 3.3 billion years old. At that time, the universe was smaller and more crowded, so galaxies collided more often, spilling gas into the supermassive black holes at galactic centers, and the universe had not converted as much of its gas into stars and, thus, was more gas-rich.

Quasar Quest

At still greater distances, which correspond to even earlier epochs, quasars are rarer. Far-off quasars are also harder to detect. “It’s a very challenging task to find such distant quasars,” says Feige Wang at the University of Arizona in Tucson. Not only do they look fainter, but expanding space also stretches their light enormously. As a result, though most quasars are intrinsically blue, remote quasars look red, lost amid the Milky Way’s many dim red stars, as well as compact ruddy galaxies beyond.

“This discovery basically puts a very extreme constraint on the black hole growing scenario. That’s why it’s special.”

—Feige Wang

In November 2019, two of Wang’s Arizona colleagues, Xiaohui Fan and Jinyi Yang, were at the Magellan telescope in Chile, hoping to find a distant quasar. “The weather was not very good,” Fan recalls. One by one, the spectra that the telescope was recording showed that the dim red specks on his list were foreground stars or galaxies, not far-off quasars. But Wang had highlighted one faint object in the constellation Eridanus that had unusual properties.

Clouds arrived just as the object was rising. “So we caught the last chance to observe it,” says Fan, who, within seconds, saw a quasar farther than any other known: Its spectral lines were shifted redward by 764%, giving it a redshift of 7.64. “Then the clouds came in,” he says. “And then we closed the telescope.”

The quasar, named J0313-1806, is 13.1 billion light-years away (4). The high speed of its orbiting gas indicates a black hole with about 1.6 billion times the mass of the Sun, 400 times that of the supermassive black hole at the center of the Milky Way. Yet the quasar shone when the universe was only 680 million years old. How did it grow so big so fast?

Quasar Queasiness

“It’s like finding a giant baby in the delivery room,” says Avi Loeb, an astronomer at Harvard University in Cambridge, MA, who was not involved in the quasar’s discovery. Wang says, “This discovery basically puts a very extreme constraint on the black hole growing scenario. That’s why it’s special.” Nor is this quasar the sole troublemaker. Eight known quasars have redshifts above 7, and at least five of them, including the record-breaker, possess black holes topping a billion solar masses (5).

“It’s a challenge to figure out how black holes can grow to such large size,” says Mitchell Begelman at the University of Colorado Boulder. One possibility is that each supermassive black hole started small. We know that a black hole with 10 to 100 solar masses can form when a large star collapses and dies. The dark body could then have grown larger by swallowing gas and stars.

But some of this infalling matter gets converted into radiation, and the resulting photons exert pressure that can impede additional gas from falling into the black hole. When the two opposing forces—the inward pull of gravity and the outward push of radiation pressure—are in balance, a situation known as the Eddington limit, a black hole doubles its mass only once every 30 million years. That’s a problem: Because the first stars and galaxies probably formed some 300 million years after the Big Bang (6), a small black hole doesn’t have enough time to grow supermassive. At this doubling rate, a 100-solar-mass black hole born then wouldn’t even attain a million solar masses by the time the universe is 680 million years old, leaving the object much too small to power a quasar.

In the 1970s, Begelman and Martin Rees at the University of Cambridge in England saw a way around the Eddington limit if the black hole gobbles enough material (7). “You’re accreting fast enough for the gas to grab onto this radiation that’s trying to escape and actually drag it towards the black hole,” Begelman says. Thus, the photons plunge into the black hole and don’t interfere with its growth.

Still, theoretical simulations of black hole growth in the early universe indicate that sustaining an accretion rate faster than the Eddington limit is difficult (8). “Black holes are messy and fussy eaters,” says Marta Volonteri at the Paris Institute of Astrophysics in France. In addition to radiation pressure, other processes push gas away. For example, nearby exploding stars can catapult gas out of the black hole’s reach, starving it. “This has been something that we keep on seeing over and over and over in simulations,” she says. On top of that, a small black hole is such a lightweight that it probably won’t sink to the center of its galaxy, where gas supplies are most abundant. As a result, there won’t be much gas around for the black hole to accrete. Thus, it may be a tall order for a small black hole to grab a billion solar masses of new material in only a few hundred million years.

A second possibility for creating the first quasars therefore starts with a black hole that’s already huge, roughly a million times the mass of the Sun. Such a black hole may have formed after a giant cloud of gas collapsed and created a supermassive star that shone briefly and then died. Once it’s born, a million-solar-mass black hole need bulk up by only a factor of a thousand to reach a billion solar masses. “Their growth is much, much easier,” Volonteri says, in part because a supermassive black hole will occupy the center of a developing galaxy whose gravity is attracting lots of stars and gas. However, no supermassive star has ever been seen. Furthermore, she says simulations suggest that even in the early universe, it is hard to create enough supermassive stars to explain all the quasars we observe (9).

So did the big black holes in distant quasars start little or large? “I’m agnostic,” says Loeb, who, as a theorist, has explored both options. “The verdict should come out from observations.” In particular, the recently launched James Webb Space Telescope can study galaxies that are even farther than the farthest quasar, allowing us to see how massive their supermassive black holes were at earlier epochs.

The Most Luminous Quasar

This year, astronomers reported another superlative quasar. “This one appears to be the most luminous object in the universe,” says Christopher Onken at the Australian National University in Canberra. It’s also the most voracious, swallowing 400 solar masses of material a year.

An earlier discovery triggered the new one. In 2022, Onken and his colleagues found that a fairly bright “star,” first photographed in 1890, was actually the most luminous quasar of the past 9 billion years (10).

“This one having slipped through our fingers for so long, we wanted to see whether there were others hiding in plain sight,” Onken says. Sure enough, there was: Located in the southern constellation Pictor, quasar J0529-4351 emits 500 trillion times the luminosity of the Sun (11)—roughly 20,000 times more than the entire Milky Way Galaxy. The quasar’s black hole is one of the largest known, with 17 billion solar masses. If put in place of the Sun, the black hole would have an event horizon—its point of no return—11 times farther out than distant Neptune.

Because of its size and brilliance, the newfound quasar offers an opportunity. The gaseous region orbiting the black hole is so big and bright that the Very Large Telescope Interferometer in Chile should soon be able to resolve it, yielding an accurate measurement of the black hole’s mass—even though the quasar is 12.2 billion light-years from Earth. “That would be very exciting,” Loeb says. Astronomers have made such measurements for much closer quasars, with smaller black holes, but this would be a first for a quasar with one of the largest black holes. That’s important, says Christian Wolf of the Australian National University, because right now, estimates of the largest black hole masses come about by extrapolating from the smaller ones. A direct measurement would indicate whether those estimates are correct or whether the largest black holes are even larger than now thought—which will exacerbate the challenge of explaining their rapid growth.

With its superb resolution and infrared sensitivity, the James Webb Space Telescope is currently scrutinizing the farthest known quasars. “We can directly see the stars in the quasar host galaxy,” Wang says. This should reveal the total mass of stars in these galaxies. Then, by comparing the stellar masses and the black hole masses of galaxies from different epochs, astronomers can see whether the two grew in tandem, a sign that the outpouring of radiation from a quasar affects star formation in the surrounding galaxy. The telescope can also see whether the galaxies containing distant quasars are loners or, instead, have other galaxies around them. If a pattern emerges, it may help explain what ignited the first and farthest quasars. If observations reveal that the quasars usually have companions, then this will imply that collisions between galaxies drive the activity in distant quasars by spilling gas into their black holes. Furthermore, such collisions could cause the central black holes to merge, boosting their mass.

Meanwhile, ground-based telescopes hunt for even farther quasars, amid hopes of reaching redshifts 8 or 9—with the potential to further challenge ideas of how the universe’s first black holes grew so rapidly.