In a first, JWST detected starlight from distant galaxies with quasars

CAMBRIDGE, MASS. — For the first time, astronomers have detected starlight from distant galaxies that host extremely bright supermassive black holes called quasars.

Data from the James Webb Space Telescope reveal that four of these galaxies are massive, compact and possibly disk-shaped, astronomers report June 12 at the JWST First Light meeting. Studying the galaxies could help solve the mystery of how black holes in the early universe grew so big so fast (SN: 1/18/21).

“Ever since the discovery of [distant] quasars, there have been studies trying to detect their host galaxies,” said MIT astrophysicist Minghao Yue. But until JWST’s sharp infrared eyes came along, it wasn’t possible. “This opens up brand new windows towards finally understanding luminous quasars and their host galaxies.”

Quasars are black holes that are feeding so furiously, the material they gobble heats to white-hot temperatures, shining brighter than the stars in the galaxy around them. They’re so bright and distant that each appears as a single, starlike point of light.

Two independent groups used that starlike quality to erase the black hole glow from images of their galaxies, like a sculptor coaxing a figure out of marble.

Yue and colleagues used JWST to observe six quasar-hosting galaxies. Around the same time, astrophysicist Xuheng Ding of the Kavli Institute for the Physics and Mathematics of the Universe in Tokyo and colleagues used JWST to look at another pair of quasars. The light from all the quasars was emitted more than 12.8 billion years ago, or less than a billion years after the Big Bang.

The teams used actual stars in the images to simulate the starlike shapes of the quasars. Then they subtracted the simulated quasar from the image of each whole galaxy, and voilà: Only starlight remained.

Ding’s team got a direct peek at both of their galaxies, while Yue’s team glimpsed two of their six. All the measured galaxies appear to be less than a tenth as wide as the Milky Way, measuring between 2,600 and 8,000 light-years across. The two galaxies that Yue and colleagues observed contain enough stars to make up between 10 billion and 100 billion times the mass of the sun, the researchers estimate. The pair that Ding and colleagues looked at weigh in at about 25 billion and 63 billion solar masses, the team reported at the meeting and in a study to appear in Nature.

Those masses are comparable to that of all the stars in the Milky Way, which in total add up to roughly 60 billion times the mass of the sun. That’s surprisingly massive for so early in the universe’s history.

What’s more, the galaxies seem to break a rule set by observations of galaxies in the nearby universe. Locally, galaxies tend to split their mass between stars and black holes in a predictable way: The more massive its central supermassive black hole, the more stars a galaxy has. These galaxies appear to pack more mass into their black hole than their amount of stars should allow.

“At least for these luminous quasars, they really are over-massive,” Yue said.

The mass calculations might prove to be overestimates, says astrophysicist Paul Shapiro of the University of Texas at Austin who was not involved in either study. Converting the light that JWST can see into stars rests on assumptions about how many stars of various masses a galaxy has. Modern galaxies have a lot more dim, lightweight stars than bright, hefty ones, so astronomers typically assume that the brightest stars they see are just the tip of the iceberg. But that might not have been the case 800 million years after the Big Bang, Shapiro says.

“You’re observing the tail and inferring the dog,” he says. “If there were a mass distribution that favors high-mass stars, you could be significantly overestimating the mass associated with the light.”

But “the fact that we can see it at all is very exciting,” says astronomer Madeline Marshall of the National Research Council Canada in Victoria. The fact that two groups are reporting starlight from quasar hosts independently is very convincing, she says.

“Pre-JWST, we could not detect host galaxies of [distant] quasars,” she said at the meeting. “Now, with only the first year of observations … we can actually detect some of these hosts for the first time.”

These first few quasar hosts are just the beginning, Ding says. JWST is scheduled to observe at least 10 more, some of which are even farther away. A larger sample will help astronomers figure out enduring cosmic riddles about how black holes and galaxies influence each other as they grow.

“We don’t know how black holes can be so big in the early universe,” Ding says. “You need to understand the environment of this monster, how it can collect so much matter to it. So knowing the conditions — the mass of the host galaxies, for example — at least then you can say how their local environment is.”

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