Webb Finds Green Pea Galaxies in Early Universe

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Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected three high-redshift galaxies with remarkable similarities to ‘green pea’ galaxies, a rare class of small galaxies in the local Universe.

A trio of faint galaxies (circled) captured in the Webb’s deep image of the galaxy cluster SMACS J0723.3-7327 exhibit properties remarkably similar to rare, small galaxies called green peas found much closer to home. Image credit: NASA / ESA / CSA / STScI.

A trio of faint galaxies (circled) captured in the Webb’s deep image of the galaxy cluster SMACS J0723.3-7327 exhibit properties remarkably similar to rare, small galaxies called green peas found much closer to home. Image credit: NASA / ESA / CSA / STScI.

Green pea galaxies were discovered and named in 2009 by volunteers taking part in Galaxy Zoo, a project where citizen scientists help classify galaxies in images.

These galaxies stood out as small, round, unresolved dots with a distinctly green shade, a consequence of both the colors assigned to different filters in the survey’s composite images and a property of the galaxies themselves.

Green pea colors are unusual because a sizable fraction of their light comes from brightly glowing gas clouds.

The gases emit light at specific wavelengths — unlike stars, which produce a rainbow-like spectrum of continuous color.

Green pea galaxies are also quite compact, typically only about 5,000 light-years across or about 5% the size of our Milky Way Galaxy.

“Peas may be small, but their star-formation activity is unusually intense for their size, so they produce bright ultraviolet light,” said Dr. Keunho Kim, a postdoctoral researcher at the University of Cincinnati.

“Thanks to ultraviolet images of green peas from Hubble and ground-based research on early star-forming galaxies, it’s clear that they both share this property.”

In July 2022, Webb taken the deepest and sharpest infrared image of the distant Universe yet seen, capturing thousands of galaxies in and behind a galaxy cluster known as SMACS J0723.3-7327.

The cluster’s mass makes it a gravitational lens, which both magnifies and distorts the appearance of background galaxies.

Among the faintest galaxies behind the cluster were a trio of compact infrared objects that looked like they could be distant relatives of green peas.

The most distant of these three galaxies was magnified by about 10 times, providing a significant assist from nature on top of the telescope’s unprecedented capabilities.

Webb’s Near-Infrared Spectrograph (NIRSpec) instrument also captured the spectra of selected galaxies in the SMACS J0723.3-7327 field.

When Dr. Kim and colleagues examined these measurements and corrected them for the wavelength stretch resulting from the expansion of space, they saw characteristic features emitted by oxygen, hydrogen, and neon line up in a stunning resemblance to those seen from nearby green peas.

Additionally, the Webb spectra made it possible to measure the amount of oxygen in these Cosmic Dawn galaxies for the first time.

As stars produce energy, they transmute lighter elements like hydrogen and helium into heavier ones.

When stars explode or lose their outer layers at the ends of their lives, these heavier elements become incorporated into the gas that forms the next stellar generations, and the process continues.

Over cosmic history, stars have steadily enriched the Universe.

Two of the Webb galaxies contain oxygen at about 20% of the level in our Milky Way.

They resemble typical green peas, which nevertheless make up less than 0.1% of the nearby galaxies observed by the Sloan survey.

The third galaxy studied is even more unusual.

“We’re seeing these objects as they existed up to 13.1 billion years ago, when the universe was about 5% its current age,” said Dr. Sangeeta Malhotra, an astronomer at NASA’s Goddard Space Flight Center.

“And we see that they are young galaxies in every sense — full of young stars and glowing gas that contains few chemical products recycled from earlier stars.”

“Indeed, one of them contains just 2% the oxygen of a galaxy like our own and might be the most chemically primitive galaxy yet identified.”

The results appear in the Astrophysical Journal Letters.

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James E. Rhoads et al. 2023. Finding Peas in the Early Universe with JWST. ApJL 942, L14; doi: 10.3847/2041-8213/acaaaf

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