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Wednesday, April 17, 2024

Astronomers May Have Just Spotted the Universe’s First Galaxies

Scientists just announced that they’ve detected what might be some of the earliest galaxies to form in the universe, a tantalizing discovery made thanks to NASA’s new flagship James Webb Space Telescope. 

“This is the first large sample of candidate galaxies beyond the reach of the Hubble Space Telescope,” astronomer Haojing Yan said yesterday at a press conference at the American Astronomical Society meeting in Seattle. Yan, who is at the University of Missouri, led the newly published study. Because the more sensitive JWST can see further into deep space than its predecessor Hubble does, it essentially sees further back in time. In the new catalog of 87 galaxies astronomers have spotted using it, some could date back to about 13.6 billion years ago, just 200 million years after the Big Bang. That’s when the galaxies emitted the light that we’re seeing today—although those systems of stars, gas, and dust would have changed dramatically since then, if they still exist at all.

While scientists have studied other faraway galaxies that date back to when the universe was still young, the discoveries by Yan and his colleagues could break those records by a few hundred million years or so. But at this point, they are all still considered “candidate galaxies,” which means that their birthdates still need confirmation. 

Dating a galaxy can be a challenging matter: It involves measuring its “redshift,” how much the light it emits is stretched toward longer red wavelengths, which tells astronomers how fast the galaxy is moving away from us in the quickly expanding universe. That, in turn, tells astronomers the galaxy’s distance from Earth—or more exactly, the distance that the photons from its stars had to travel at the speed of light before reaching a space telescope near the Earth, like JWST. Light from stars in the most distant galaxy in this collection may have been emitted 13.6 billion years ago, likely fairly soon after the young galaxy came together. 

These newly estimated distances will have to be confirmed with spectra, which means measuring the light the galaxies emit across the electromagnetic spectrum and pinpointing its unique signatures. Still, Yan expects many of them to be correctly dated to the early days of the cosmos: “I’ll bet $20 and a tall beer that the success rate will be higher than 50 percent,” he said.

Yan’s team imaged these galaxies with JWST’s NIRCam at six near-infrared wavelengths. To estimate their distances, the astronomers used a standard “dropout” technique: Hydrogen gas surrounding galaxies absorbs light at a particular wavelength, so the wavelengths at which an object can or can’t be seen puts a limit on how far away it is likely to be. These 87 candidate galaxies mostly look like blobs that can only be detected in the longer (and therefore redder) near-infrared wavelengths detectable by NIRCam, which could mean they’re very distant, and therefore very old. 

However, it’s possible that some of them could be much closer than expected—which would mean they aren’t so old after all. For example, it could be that their light is just too faint to be detected at some wavelengths. Until Yan can collect more detailed data, he won’t know for sure.

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Many astronomers have been excited to use the new telescope to investigate early-universe galaxies, vast metropolises of millions and sometimes billions of stars. Each galaxy takes a long time to develop its unique shape: Many look sort of like a sombrero, with a bulgy inner part and a thinner disk of galaxies beyond that, while others only have the round, bulgy part. Astronomers previously thought few galaxies in the early universe had disks, but it turns out observers just couldn’t see dim ones before Webb came on the scene. 

Now, some astronomers are wondering if early galaxies might have complex structures like our own Milky Way’s huge spiral arms, says Jeyhan Kartaltepe, an astrophysicist at the Rochester Institute of Technology who’s part of the Cosmic Evolution Early Release Science survey and also presented work at the astronomy conference. “The increased resolution of JWST allows us to see the structure more clearly, and the high sensitivity of the NIRCam instrument allows us to see faint features that we just couldn’t see before,” she says.

Kartaltepe and her team examined 850 galaxies with Webb’s infrared camera. These galaxies date to between about 11.5 and 13 billion years in the past, and her group found that around 40 percent of them have disks. Their new study is currently going through peer review.

Another team of astronomers who used JWST’s Near Infrared Spectrograph, which measures the intensity of light over a range of wavelengths, also announced new research results yesterday: They found three objects dating to about 700 million years after the Big Bang that are akin to the small “green pea” galaxies people have already glimpsed in our nearby universe. (The moniker comes from citizen science volunteers with the Galaxy Zoo project, because the false-color images they use from the Sloan Digital Sky Survey to classify these galaxies make them look round and green.) 

James Rhoads, an astrophysicist at NASA Goddard Space Flight Center, led this team, and their work was also published last week. “These are among the first three spectra from the cosmic dawn of the universe from JWST,” he said at yesterday’s press conference.

His team’s research suggests that 700 million years after the universe exploded into existence, these kinds of compact galaxies—which are thought to be young and forming stars—were probably common. We still sight similar galaxies today in nearby space, he says, but “green pea galaxies are much rarer in our cosmic backyard.” 

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