Astronomers observe elusive stellar light surrounding ancient quasars

The observations suggest some of earliest “monster” black holes grew from massive cosmic seeds.


The blazing centers of active galaxies are known as quasars. As a result of their ability to consume massive amounts of matter over eons longer than previously thought, quasars are among the universe’s most brilliant phenomena. This ring is incredibly bright and long-lasting.

Quasars outshine the rest of their galaxy due to their extreme brightness. However, the MIT team finally saw the much fainter light from stars in the host galaxies of three ancient quasars.

Astronomers observed mysterious brightness enveloping some of the universe‘s oldest quasars. The distant signals, which date back more than 13 billion years to the early cosmos, are providing hints about the evolution of the first black holes and galaxies.

The study sheds light on how the earliest supermassive black holes became so massive despite having a relatively short amount of cosmic time to grow.

Study author Minghao Yue, a postdoc at MIT‘s Kavli Institute for Astrophysics and Space Research, said, “After the universe came into existence, there were seed black holes that consumed material and proliferated. One big question is understanding how those monster black holes could grow so quickly.”

Study author Anna-Christina Eilers, assistant professor of physics at MIT, said, “Our results imply that in the early universe, supermassive black holes might have gained their mass before their host galaxies did, and the initial black hole seeds could have been more massive than today.”

Astronomers previously believed that the quasar’s light stemmed from a single, star-like “point source.” Hence, they designated quasars as a portmanteau of a “quasi-stellar” object.

After first observations, scientists have realized that quasars are not stellar in origin but emanate from the accretion of intensely powerful and persistent supermassive black holes sitting at the center of galaxies that also host stars, which are much fainter in comparison to their dazzling cores.

Distinguishing the light from the central black hole of a quasar from the starlight of the host galaxy has proven challenging. It’s similar to distinguishing between a swarm of fireflies and a large, central searchlight. But with the advent of NASA’s James Webb Space Telescope (JWST), which can look back in time farther and with much higher sensitivity and precision than any other telescope, scientists now stand a lot better chance of doing so than they had before.

In this study, astronomers used dedicated time on JWST to intermittently observe six known ancient quasars from the fall of 2022 through the following spring. The team collected more than 120 hours of observations of the six distant objects.

In this investigation, six known, old quasars were observed sporadically from the fall of 2022 through the spring of 2023 using specially designated time on JWST. The crew followed the six far-off objects for more than 120 hours.

Yue says, “The quasar outshines its host galaxy by orders of magnitude. And previous images were not sharp enough to distinguish what the host galaxy with all its stars looks like. Now, for the first time, we can reveal the light from these stars by carefully modeling JWST’s much sharper images of those quasars.”

The group evaluated the JWST imaging data of all six far-off quasars, estimating their ages to roughly 13 billion years. These data contained wavelength-specific observations of each quasar’s radiation. Using that data, scientists created a model to predict how much of the light is most likely coming from a compact “point source,” like the accretion disk of a core black hole, as opposed to a more diffuse source, like light from the scattered stars that surround the host galaxy. 

Using this modeling, the scientists could distinguish between the light from the more diffuse stars in the host galaxy and the light from the bright disk of the central black hole in each quasar. Both sources’ entire mass is reflected in the light they emit. The mass ratio of the central black hole to the host galaxy’s mass was estimated to be approximately 1:10 for these quasars. They saw that this stood in sharp contrast to the mass balance of 1:1,000 observed today when more recent black holes are far less massive than their host galaxies.

“This tells us something about what grows first: Is it the black hole that grows first, and then the galaxy catches up? Or is the galaxy and its stars that first grow, and they dominate and regulate the black hole’s growth?” Eilers explains. “We see that black holes in the early universe grow faster than their host galaxies. That is tentative evidence that the initial black hole seeds could have been more massive back then.”

“There must have been some mechanism to make a black hole gain their mass earlier than their host galaxy in those first billion years,” Yue adds. “It’s kind of the first evidence we see for this, which is exciting.”

The findings are published today in The Astrophysical Journal


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