Using the James Webb Space Telescope (JWST), astronomers have discovered star clusters in the "Cosmic Gems" arc that existed just 460 million years after the Big Bang. This marks the first discovery of star clusters in an infant galaxy, seen as it was when the 13.8 billion-year-old universe was less than 500 million years old.
The Cosmic Gems arc, initially discovered by the Hubble Space Telescope and officially designated SPT0615-JD1, is a gravitationally lensed infant galaxy around 13.3 billion light-years from Earth. That means light from this galaxy, seen by the JWST, has been traveling to Earth for around 97% of the universe's lifetime.
The international team of astronomers behind this discovery found five young massive star clusters in the Cosmic Gems arc. These clusters existed during a period when young galaxies were undergoing intense bursts of star formation and emitting huge amounts of ultraviolet light. This radiation may be responsible for triggering one of two major phases in the evolution of the universe: the epoch of cosmic reionization.
Studying these five-star clusters could teach astronomers a great deal about this early period in the cosmos.
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"The surprise and astonishment were incredible when we opened the JWST images for the first time," Angela Adamo of Stockholm University and the Oskar Klein Centre in Sweden, and team leader, said in a statement. "We saw a little chain of bright dots mirrored from one side to the other — these cosmic gems are star clusters! Without the JWST, we would not have known we were looking at star clusters in such a young galaxy!"
The newly detected star clusters in the Cosmic Gems arc are remarkable because of their massive and dense nature. The density of the five star clusters is considerably greater than that of nearby star clusters.
A helping hand from Einstein
The epoch of reionization is so important because this was the stage at which the first light sources in the cosmos — early galaxies, stars, and feeding supermassive black hole-powered quasars — supplied the energy that split electrons away from neutral hydrogen that filled the universe.
The newly found star clusters are located in a very small region of their galaxy but are responsible for the majority of ultraviolet light coming from that galaxy, meaning clusters like these may have been the primary drivers of reionization.
By studying reionization, scientists can learn more about the processes that formed large-scale structures in the universe. This can reveal how the remarkably smooth matter distribution during early cosmic times gave way to the highly structured universe of galaxies (and clusters of galaxies) astronomers see in the universe's later epochs.
More specifically, these five early star clusters can show where stars formed and how they were distributed during the infancy of the cosmos. This offers a unique opportunity to study star formation as well as the inner workings of infant galaxies at an unprecedented distance, the study team says.
"The JWST's incredible sensitivity and angular resolution at near-infrared wavelengths, combined with gravitational lensing provided by the massive foreground galaxy cluster, enabled this discovery," Larry Bradley, the principal investigator of the observing program that captured these data, said in the statement. "No other telescope could have made this discovery."
To see such distant objects as they existed in the early universe, the JWST uses a principle from Einstein's 1915 theory of gravity: general relativity.
General relativity suggests objects with mass cause the very fabric of space and time, united as a four-dimensional entity called "spacetime," to warp. The more mass an object has, the greater the warping of spacetime it causes.
When light from background sources passes this warping, its path gets curved. The closer the light passes to the warping object, the more its path gets curved. As a result of this, light from a single object can arrive to an observer, like the JWST, more than once and at different times.
That means light sources can appear in multiple places in the same image, have their positions shifted to apparent positions, or, most usefully, have their light amplified. The latter phenomenon is called "gravitational lensing," with the body between a distant background object and Earth called a "lensing object."
In this case, the lensing object is a lensing galaxy cluster called SPT-CL J0615−5746, and the background objects are the Cosmic Gems, their star clusters, and two distant lensed galaxies.
"What is special about the Cosmic Gems arc is that thanks to gravitational lensing, we can actually resolve the galaxy down to parsec scales!" Adamo said.
How do globular clusters come together?
One promising follow-up study to come from this JWST observation of early star clusters relates to how arrangements of stars, called "globular clusters," are formed. As seen in our galaxy, the Milky Way, globular clusters are ancient relics of intense bursts of star formation in the early universe.
Scientists aren't entirely sure how these spherical conglomerations of tightly packed, gravitationally bound stars come together, but it may be key that massive and dense young star clusters in the Cosmic Gems arc could well be the beginning stages of globular cluster formation. This means they could provide an incredibly useful window into the early stages of globular cluster birth.
These five star clusters could also help understand other aspects of cosmic evolution.
"The high stellar densities found in the clusters provide us with the first indication of the processes taking place in their interiors, giving new insights into the possible formation of very massive stars and black hole seeds, which are both important for galaxy evolution," Adamo said.
The study of the Cosmic Gems arc will continue with the team behind this research already planning to observe this early galaxy with the JWST's Near Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI) instruments during Cycle 3 of the $10 billion space telescope's operations.
"The NIRSpec observations will allow us to confirm the redshift of the galaxy and to study the ultraviolet emission of the star clusters, which will be used to study their physical properties in more detail," Bradley said. "The MIRI observations will allow us to study the properties of ionized gas."
These spectroscopic observations should reveal just how intense star formation was in the active sites of this infant galaxy.
The astronomers behind this study also now intend to study other galaxies to hunt for star clusters similar to these five.
"I am confident there are other systems like this waiting to be uncovered in the early universe, enabling us to further our understanding of early galaxies," team member Eros Vanzella from the National Institute for Astrophysics (INAF) said.
The team's research was published on Monday (June 24) in the journal Nature.
