The James Webb hyperlinks fashionable inexperienced pea galaxies with historic galaxies within the cosmic daybreak
When the James Webb Space Telescope lifted off Earth on Christmas Day 2021, it brought with it many expectations. One of his scientific goals is to seek the light of the first galaxies in the universe and to study how galaxies form and evolve.
A new paper shows that JWST is doing just that and has found a connection between the first galaxies and rare galaxies in our backyard that astronomers are calling “Green Pea” galaxies.
One of the big questions in cosmology is how galaxies first formed and then evolved into the giant spiral structures we see today. The appearance of the first galaxies is associated with the Epoch of Reionization (EOR) and the end of the Dark Ages of the Universe. Before the first stars and galaxies formed in the universe, there was nothing but an impenetrable veil of primordial gas. After stars formed in galaxies, their intense ultraviolet light reionized hydrogen, allowing the light to travel across the universe.
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The evidence for the EOR is buried in the deep, distant past. The light from these early galaxies has since been stretched to extreme redshifts by the expansion of the Universe, and the JWST has been carefully designed to detect this infrared light. It was successful, and at the start of its scientific mission, JWST has already discovered the oldest galaxies ever found, dating only 350 million years after the Big Bang.
A new article published in The Astrophysical Journal Letters shows how some of these very early galaxies are remarkably similar to some strange galaxies that are much younger and live in our cosmic backyard. The paper is Finding Peas in the Early Universe with JWST. The lead author is James Rhoads, an astrophysicist at NASA’s Goddard Space Flight Center.
“With detailed chemical fingerprints of these early galaxies, we see that they contain perhaps the most primitive galaxy identified to date,” Rhoads said in a press release. “At the same time, we can connect these galaxies from the beginning of the universe with similar ones nearby, which we can study in much more detail.”
Green peas are a type of galaxy first identified in the Citizen Science Galaxy Zoo Project. In this project, citizen scientists set out to identify galaxies in the large amounts of imagery generated by surveys such as the Sloan Digital Sky Survey. The Green Pea galaxies were notable because they are small, round dots that are difficult to resolve. They are also green, partly because of the filtering of the images and partly because of their metallicity or chemical composition.
But green peas are also green because they host very active star formation. Unlike stars, which emit light across the entire electromagnetic spectrum, green peas’ light comes from clouds of glowing gas illuminated by starlight. So instead of seeing the unfiltered starlight, we only see the light from the energized gas clouds.
“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.
A new analysis of distant galaxies imaged by NASA’s James Webb Space Telescope shows they are extremely young and share some striking similarities with “green peas,” a rare class of small galaxies in our cosmic backyard. Image credits: NASA, ESA, CSA and STScI
Green peas are only about 5,000 light-years across. Compare that to most modern galaxies like the Milky Way, which is about 85,000 light-years across. Green Pea galaxies are clearly strange, and astronomers are very interested in them.
Astronomers are also interested in the earliest galaxies, and James Webb has excelled in finding them. The authors of this study examined data from the JWST Early Release Observations (ERO). That data included the sharpest and deepest infrared image of the Universe ever recorded. The image focused on a galaxy cluster called SMACS 0723, which acted as a gravitational lens, enhancing the JWST’s already impressive range. The gravitational lens magnified the light from galaxies beyond the galaxy cluster.
Among those galaxies, astronomers identified three that shared characteristics with Green Peas. The light from one of them was amplified tenfold by the galaxy cluster, a remarkable combination of natural circumstances that enhanced the JWST’s impressive power.
These images show a green pea galaxy on the left, captured by the Sloan Digital Sky Survey, and an infrared image of an early pea captured by NASA’s James Webb Space Telescope. At left is J122051+491255, a green pea about 4,000 light-years across about 170 million light-years away. This is a typical size for a green pea. At right is an early pea called 04590, whose light took 13.1 billion years to reach us. 04590 is even more compact than the other two JWST galaxies, comparable to the smallest green peas nearby. Image credits: SDSS and NASA, ESA, CSA and STScI
Part of the JWST’s power lies in its near-infrared spectrograph (NIRSpec) instrument. NIRSpec can observe the spectra of hundreds of objects simultaneously. When Rhoads and his colleagues examined the spectra of all the galaxies in the image, they found the three green pea-like ones. Green peas feature oxygen, hydrogen and neon emissions, and the team found the same properties, or chemical fingerprints, in the three ancient, distant galaxies.
This image is based on data from the publication, but has been emphasized to make it easier for non-specialists to read. JWST’s NIRSpec instrument captured the chemical fingerprints of selected galaxies behind SMACS 0723, including three faint, distant objects. Corrected for the wavelength expansion caused by the billions of years of space expansion, the spectra of these galaxies (shown in red) exhibit features emitted by oxygen, hydrogen, and neon that bear a striking resemblance to those emitted seen from so-called green, pea galaxies are found nearby (in green). In addition, the Webb observations made it possible for the first time to measure the amount of oxygen in these cosmic dawn galaxies. The spectral lines have been stretched vertically to show these relationships. Photo credit: NASA’s Goddard Space Flight Center/Rhoads et al. 2023
Something unusual happens when galaxies about 13 billion years old have the same chemical fingerprint, or metallicity, as much younger galaxies. That’s because metallicity increases as the universe ages. Metallicity, which in astronomy refers to chemical elements heavier than hydrogen and helium, increases as subsequent stars live and die. Stars create heavier elements through nucleosynthesis, and when the stars die, they disperse these metals into space to be included in the next generation of star formation.
“We see these objects as they existed 13.1 billion years ago, when the universe was about 5% of its current age,” said Goddard researcher Sangeeta Malhotra. “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 previous stars. In fact, one of them contains only 2% of the oxygen of a galaxy like our own and may be the most chemically primitive galaxy identified to date.”
The oxygen content plays an important role in the comparison. Two of the three galaxies contain only 20% as much oxygen as the Milky Way, and one contains only 2%. Their oxygen content is very similar to that of green peas. Why would modern green pea galaxies have the same metallicity as the universe’s first galaxies?
Part of the answer lies in the green peas. In terms of metallicity – especially oxygen content – and star formation, they are low-redshift analogues of the three high-redshift galaxies in the JWST image. Because they are so much closer, they are much easier to observe and study.
“Low redshift analogues for reionization epoch galaxies have been of tremendous value in recent years, allowing us to probe properties of nearby objects that could not be probed directly in faint redshift galaxies at Cosmic Dawn,” they write authors in their paper. Their work shows how the powerful JWST can tease out important details in emissions from faint, redshifted galaxies. Although these results are based on the JWST early release operations, future observations will provide more detail about the old galaxies. “This ultimately includes metallicity, temperature, ionization parameters, density and gas pressure,” they explain.
However, some features of these early galaxies remain hidden due to the obscuring effect of extreme redshifts. Some of these traits are more easily observed in green peas. “For example, radio emission from atomic gas can be studied in individual galaxies in Green Peas, but is far out of reach in the early Universe,” they explain. Among other things, these radio emissions can tell astronomers more about the hydrogen and star formation in the galaxies.
Finding connections between green peas and ancient, strongly redshifted galaxies is a major development in cosmology. This means observations from Green Peas can help explain the early galaxies. “As a result, JWST now enables us to determine the validity of local analog populations with unprecedented accuracy and confidence, paving the way for further advances using both the most distant and the closest young galaxies,” the researchers explain.
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