A new analysis of distant galaxies imaged by NASA’s James Webb Space Telescope shows that they are extremely young and share some remarkable similarities with “green peas,” a rare class of small galaxies in our cosmic backyard.
“With detailed chemical fingerprints of these early galaxies, we see that they comprise what may be the most primitive galaxy yet identified. At the same time, we can connect these galaxies from the dawn of the universe to similar ones nearby that we can study in much greater detail,” said James Rhodes, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who presented the findings at 241 -th meeting of the American Astronomical Society in Seattle.
A paper describing the results, led by Rhoads, was published Jan. 3 in The Astrophysical Journal Letters.
The green pea galaxies were discovered and named in 2009 by volunteers involved in Galaxy Zoo, a project in which citizen scientists help classify galaxies in images, starting with those from the Sloan Digital Sky Survey. The peas stood out as small, round, unresolved dots with a distinctly green tint, a consequence both of the colors assigned to various filters in the survey’s composite images and a property of the galaxies themselves.
The colors of pea-green galaxies are unusual because a significant portion of their light comes from brightly glowing gas clouds. Gases emit light at specific wavelengths—unlike stars, which produce a rainbow-like spectrum of continuous color. The pea is 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-forming activity is unusually intense for their size, so they produce bright ultraviolet light,” said Keunho Kim, a postdoctoral researcher 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 is clear that both share this property.”
In July 2022, NASA and its Webb mission partners released the deepest and sharpest infrared image of the distant universe ever seen, capturing thousands of galaxies in and behind a cluster known as SMACS 0723. The cluster’s mass makes it a gravitational lens that, as both magnifies and distorts the appearance of background galaxies. Among the faintest galaxies behind the cluster were three compact infrared objects that looked like distant relatives of green peas. The most distant of these three galaxies was magnified about 10 times, providing significant help from nature in addition to the telescope’s unprecedented capabilities.
Webb did more than image the cluster—his Near Infrared Spectrograph (NIRSpec) instrument also captured the spectra of selected galaxies in the scene. When Rhodes and his colleagues examined these measurements and corrected them for the stretching of the wavelength due to the expansion of space, they saw characteristic features emitted by oxygen, hydrogen and neon line up in striking similarity to those seen from the nearby green peas.
In addition, Webb’s spectra made it possible to measure the amount of oxygen in these cosmic galaxies for the first time.
As stars produce energy, they transform lighter elements such as hydrogen and helium into heavier ones. When stars explode or lose their outer layers at the end of their lives, these heavier elements are incorporated into the gas that forms subsequent generations of stars, and the process continues. Throughout cosmic history, stars have continuously enriched the universe.
Two of the Webb galaxies contain oxygen at about 20% of the level in our Milky Way. They look like a typical green pea, yet make up less than 0.1% of the nearby galaxies observed by the Sloan survey. The third galaxy studied is even more unusual.
“We see these objects as they existed 13.1 billion years ago, when the universe was about 5 percent of its current age,” said Sangita Malhotra, a Goddard researcher. “And we see that they are young galaxies in every sense – full of young stars and glowing gas that contains little chemical products recycled from earlier stars. Indeed, one of them contains only 2% of the oxygen of a galaxy like ours and may be the most chemically primitive galaxy yet identified.”
NIRSpec was created for ESA (European Space Agency) by Airbus Industries. Its array of nearly half a million microshutters—tiny doors that can be opened or closed to let in or block light—allow it to capture spectra from up to 100 individual objects at once. The microshutter array and detector subsystems were manufactured by NASA.
The James Webb Space Telescope, an international mission led by NASA with its partners ESA and CSA (Canadian Space Agency), is the world’s leading space science observatory. NASA Headquarters manages the mission for the agency’s Science Mission Directorate. NASA’s Goddard Space Flight Center operates Webb for the agency and oversees mission work by the Space Telescope Science Institute, Northrop Grumman and other mission partners. In addition to Goddard, several NASA centers contributed to the project, including the Johnson Space Center in Houston, the Jet Propulsion Laboratory in Southern California, the Marshall Space Flight Center in Huntsville, Alabama, the Ames Research Center in California’s Silicon Valley, and others.
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