In the deep view of the galaxy cluster SMACS 0723 taken by the James Webb Space Telescope, three small, dim objects (circled) have characteristics that are strikingly comparable to those of unusual, nearby galaxies known as "green peas." Because of its mass, the cluster acts as a gravitational lens, enlarging and warping the image of background galaxies. We imagine these ancient peas to have existed when the cosmos was only 5% as old as it is now, 13.8 billion years ago. The furthest pea, on the left, maybe the most chemically primitive galaxy yet discovered because it only has 2% the oxygen abundance of our own galaxy.
Credits: NASA, ESA, CSA, and STScI.
January 09, 2023
Volunteers working on Galaxy Zoo, a project where citizen scientists help categorize galaxies in photographs starting with those from the Sloan Digital Sky Survey, made the discovery and naming of green pea galaxies in 2009. Small, rounded, unresolved specks with a particular green hue that were distinguished as peas in the survey's composite images as well as a characteristic of the galaxies themselves.
Because a significant portion of the light in green pea galaxies comes from brilliantly blazing gas clouds, these galaxies have distinctive colors. Unlike stars, which produce a rainbow-like spectrum of continuous color, gases emit light at specific wavelengths. Peas are typically only 5,000 light-years across, or roughly 5% the size of our Milky Way galaxy. They are also rather small.
The deepest and brightest infrared image of the distant universe ever seen was released in July 2022 by NASA and its Webb mission collaborators. It showed thousands of galaxies in and behind the SMACS 0723 galaxy cluster. Because of its mass, the cluster acts as a gravitational lens, enlarging and warping the image of background galaxies. A trio of compact infrared objects that appeared to be distant relatives of green peas were among the weakest galaxies behind the cluster. The most distant of these three galaxies was magnified by roughly ten times, which was made possible due to Webb Telescope's unmatched capabilities.
In addition to taking pictures of the cluster, Webb also used its Near-Infrared Spectrograph (NIRSpec) instrument to record the spectra of a few of the nearby galaxies. The typical features released by oxygen, hydrogen, and neon lined up in a striking resemblance to those seen from neighboring green peas when the observation team studied these measurements and corrected them for the wavelength stretch caused by the expansion of space.
The Webb spectra also allowed for the first measurement of the oxygen content in these cosmic dawn galaxies. Lighter elements like hydrogen and helium are converted into heavier ones by stars as they generate energy. These heavier elements are integrated into the gas that creates the following stellar generations after stars burst or shed their outer layers at the end of their lives, continuing the process. Over the course of the universe's history, stars have steadily improved it. Two of the Webb galaxies had oxygen levels that are about 20% lower than those of the Milky Way. Despite making up fewer than 0.1% of the nearest galaxies seen by the Sloan survey, they resemble ordinary green peas. Even more peculiarity is present in the third analyzed galaxy.