Rare ‘green pea’ galaxy may be the most ‘chemically primitive’ galaxy ever discovered

A new analysis of distant galaxies taken by the James Webb Space Telescope (JWST) shows they share characteristics with a rare class of galaxies called “green peas” found in our cosmic backyard.

One of these galaxies, which existed when the universe was only 5% of its current age, may be one of the most “chemically primitive” galaxies astronomers have ever seen.

“With detailed chemical fingerprints of these early galaxies, we see that they include what may be the most primitive galaxy identified so far,” study leader James Rhodes, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland, said in a statement. . “At the same time, we can connect these galaxies at the dawn of the universe with similar ones nearby, which we can study in much more detail.”

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Green pea galaxies were discovered during the 2009 Sloan Digital Sky Survey. Green peas are so named because they look like small, round, unresolved dots with a distinct green tint. They appear green because most of the light from these rare galaxies comes from bright, glowing gas clouds emitting light at specific wavelengths, rather than from the wide spectrum of light and continuous colors emitted by stars in other galaxies.

These green pea galaxies are rare, making up only 0.1% of nearby galaxies. They are also compact (by cosmic standards), measuring only 5,000 light-years in diameter—only 5% the width of our Milky Way galaxy. But what green-pea galaxies lack in size, they seem to make up for in star birth rates.

“Peas may be small, but their star formation activity is extraordinarily intense for their size, which is why they emit bright ultraviolet light,” said Keunho Kim, a research fellow at the University of Cincinnati and a member of the analysis team. . “Thanks to Hubble’s ultraviolet images of green peas and ground-based studies of early star-forming galaxies, it’s clear that they both share this property.”

JWST image of dim, distant galaxies resembling rare galaxies known as “green peas”. (Image credit: NASA, ESA, CSA and STScI)

Green peas in the early universe

In July 2022, the JWST team unveiled the deepest and sharpest infrared image of the distant universe ever taken, showing galaxies inside and behind the galaxy cluster known as SMACS 0723.

As a result of a phenomenon called gravitational lensing, SMACS 0723 enlarges and distorts the appearance of the galaxies behind it. The image revealed three infrared objects that resemble distant relatives of local green pea galaxies.

The gravitational lensing effect of SMACS 0723 magnified the most distant of these galaxies by a factor of 10, giving the space telescope a huge natural boost for observations.

Using its Near Infrared Spectrograph (NIRSpec) instrument, JWST also acquired spectra of the galaxies in the image, which revealed signature emissions of oxygen, hydrogen and neon, further enhancing the similarity to green pea galaxies.

Enhanced image of faint, distant galaxies resembling rare galaxies known as “green peas”. (Image credit: NASA, ESA, CSA and STScI)

This spectrographic data allowed the team to measure the amount of oxygen in these distant and early galaxies for the first time, showing that two of these galaxies contain about 20% oxygen compared to the Milky Way.

As stars die, they enrich the universe with the heavy elements they forged during their lives, meaning that early galaxies like this must have a relative deficit of elements heavier than hydrogen and helium, which astronomers call “metals.” compared to older galaxies like ours. own.

“We see these objects as they existed before 13.1 billion years ago, when the universe was about 5% older than its current age,” Sangita Malhotra, a NASA Goddard researcher and member of the research team, said in a statement. “And we see that these are young galaxies in every sense – full of young stars and luminous gas that contains little chemical products recycled from earlier stars.”

However, the third of these early lensed galaxies is more unusual. “One of them contains only 2% of the oxygen of a galaxy like ours and may be the most chemically primitive galaxy ever identified,” Malhotra said.

The team’s research was published in January. 3 in The Astrophysical Journal Letters and was presented at the 241st meeting of the American Astronomical Society in Seattle on January 2. nine.

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