What was little more than a bright spot for the Hubble Space Telescope has been revealed as one of the oldest galaxies ever discovered – and the find is thanks to none other than Hubble’s younger sibling: the James Webb Space Telescope.
The James Webb Space Telescope’s international ‘Glass’ Collaboration has made detailed observations of the galaxy, called Gz9p3, seen as it occurred just 510 million years after the Big Bang. That was during the relative infancy of the universe, which is now 13.8 billion years old.
The team found that Gz9p3, like other early galaxies observed by the JWST, is much more massive and mature than expected for a galaxy in the early universe. During the ancient period in which it was observed, it was found to contain several billion stars.
When it comes to the cosmic mystery of how early galaxies grew so big so quickly, Gz9p3 could be a real puzzle. Not only is it more massive than expected, but it is also about 10 times more massive than other galaxies the JWST has seen at similar epochs in the universe’s history.
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“Just a few years ago, Gz9p3 appeared as a single point of light through the Hubble Space Telescope,” wrote Kit Boyett, team member and scientist at the University of Melbourne, for the institute’s Pursuit publication. “But using the JWST we were able to observe this object as it was 510 million years after the Big Bang, about 13 billion years ago.”
Gz9p3 is simply remarkable. In addition to its size and maturity, its shape also reveals clues to its creation.
Was Gz9p3 created by an early galaxy merger?
Using the JWST and direct imaging, the team was able to determine that Gz9p3 has a complex shape with two bright spots revealing the two dense nuclei. That indicates that Gz9p3 likely formed when two early galaxies in the early universe collided. This collision may have been ongoing at the time astronomers spotted Gz9p3 with the JWST.
“The JWST image of the galaxy shows a morphology typically associated with two interacting galaxies. And the merger is not yet complete because we still see two components,” Boyett explains. “When two massive objects come together in this way, they essentially throw away some of the matter. So this thrown out matter suggests that what we observed is one of the most distant mergers ever seen.”
In addition to determining the age, mass and shape of this ancient galaxy, Boyett and colleagues were able to probe deeper into Gz9p3 to investigate the stellar population of these colliding galaxies. Because young stars are brighter than their older counterparts, they usually dominate images of galaxies, especially those stars so far away that their light has been traveling to Earth for billions of years.
“For example, a young, bright population created by galaxy mergers that is less than a few million years old will outcompete an older population that is more than 100 million years old,” Boyett continued.
The Glass collaboration got around this by making spectroscopic observations of Gz9p3 and using direct imaging. Spectroscopy can be used to determine the elements that make up stars; Because young and old stars have different compositions, the researchers were able to separate the two categories in this early galaxy.
Older stars have been working their way through the hydrogen supply in their cores, already melting it all into helium and then fusing that helium together to create even heavier elements, which astronomers call “metals.” This means that older stars are richer in metals than younger stars, which are still dominated by hydrogen and some helium.
The research team used the JWST to detect specific elements in the older star population of Gz9p3. These target elements include silicon, carbon and iron, the latter of which is the heaviest element that can be synthesized by stars. This means that when these stars died in supernova explosions, they would have enriched the early universe with metals. Much of this metal content would have become the building blocks of the next generation of stars.
Furthermore, the team found that the population of old stars in Gz9p3 was much larger than previously suspected. This means that while astronomers are aware of this cycle of stellar life and death and the increasing metal enrichment of subsequent generations of stars, the Gz9p3 observations indicate that galaxies may have “chemically matured” more quickly than previously suspected.
“These observations provide evidence of rapid, efficient buildup of stars and metals in the immediate aftermath of the Big Bang, coupled with continued galaxy mergers, showing that massive galaxies with several billion stars existed earlier than expected,” Boyett wrote.
A history of violence
Galaxies isolated from their galactic counterparts do form stars, but the process is slow and ends when that galaxy exhausts its reservoir of gas and dust, the materials from which stars are formed.
For galaxies that are close together, the star formation process can be accelerated and even revived after stalling. That’s because when these galaxies are pulled together by a mutual attraction, they collide. The merger then causes an influx of fresh gas, ushering in a period of rapid star birth known as a starburst. This means that mergers are an excellent way for galaxies to rapidly grow their star populations.
Most large galaxies in the universe grew this way; our own galaxy, the Milky Way, itself shows a history of mergers. For example, it has been involved in the cannibalization of smaller satellite galaxies that once orbited it. The Milky Way is currently forming stars at a stunted rate, but this will change when it collides with our neighboring galaxy, Andromeda, in about 4.5 billion years. This will cause an influx of gas that will cause another burst of fireworks.
Observations of Gz9p3 tell astronomers that this channel for rapid mass accumulation and star birth was a bigger factor in the early universe than predicted.
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“These observations of Gz9p3 show that galaxies in the early Universe could rapidly accumulate mass through mergers, with star formation efficiency being higher than we expected,” Boyett explains. “These and other observations using the JWST are causing astrophysicists to adjust their modeling of the universe’s early years.
“Our cosmology isn’t necessarily wrong, but our understanding of how quickly galaxies formed probably is, because they are more massive than we ever thought possible.”
The team’s research was published March 7 in the journal Nature Astronomy.