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 is the first discovery of star clusters in a young galaxy that look as they did when the 13.8 billion-year-old Universe was less than 500 million years old.
The Cosmic Gems arc, originally discovered by the Hubble Space Telescope and officially designated SPT0615-JD1, is a gravitationally lensed young galaxy located about 13.3 billion light-years from Earth. This means that the light from this galaxy observed by the JWST has been traveling to Earth for about 97% of the lifetime of the universe.
The international team of astronomers behind this discovery has discovered five young, massive star clusters in the Cosmic Gems arc. These clusters formed at a time when young galaxies were experiencing intense bursts of star formation and emitting large amounts of ultraviolet light. This radiation may be responsible for the beginning of one of two major phases in the evolution of the Universe: the epoch of cosmic reionization.
By studying these five-star clusters, astronomers could learn a lot about this early period of the cosmos.
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“The surprise and amazement were incredible when we opened the JWST images for the first time,” said Angela Adamo of Stockholm University and the Oskar Klein Centre in Sweden and leader of the team in a statement. “We saw a small chain of bright dots mirrored from one side to the other – these cosmic jewels are star clusters! Without the JWST, we would not have known we were looking at star clusters in such a young galaxy!”
The newly discovered star clusters in the Cosmic Gems arc are remarkable for their mass and density. The density of the five star clusters is considerably greater than that of the neighboring star clusters.
A helping hand from Einstein
The era of reionization is important because it was during this period that the first light sources in the cosmos – early galaxies, stars, and quasars powered by supermassive black holes – provided the energy needed to split electrons from neutral hydrogen in the universe.
The newly discovered star clusters lie in a very small region of their galaxy, but are responsible for most of the galaxy’s ultraviolet light, meaning that such clusters may have been the main drivers of reionization.
By studying reionization, scientists can learn more about the processes that formed large-scale structure in the universe. This can reveal how the remarkably uniform distribution of matter in early cosmic times gave way to the highly structured universe of galaxies (and galaxy clusters) that astronomers see in the later epochs of the universe.
More specifically, these five early star clusters can reveal where stars formed and how they were distributed during the infancy of the cosmos. This offers a unique opportunity to study star formation and the inner workings of young galaxies from an unprecedented distance, the study team says.
“The incredible sensitivity and angular resolution of the JWST at near-infrared wavelengths, combined with the gravitational lensing of the massive galaxy cluster in the foreground, made this discovery possible,” Larry Bradley, the principal investigator of the observing program that collected this 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 assumes that objects with mass bend the fabric of space and time, which is combined into a four-dimensional entity called “spacetime.” The more mass an object has, the greater the curvature of spacetime it causes.
When light from background sources passes through this distortion, its path becomes curved. The closer the light passes to the distorting object, the more its path becomes curved. As a result, light from a single object can reach an observer, such as the JWST, more than once and at different times.
This means that light sources can appear in multiple places in the same image, their positions can be shifted to apparent positions, or, most usefully, their light can be amplified. The latter phenomenon is called “gravitational lensing,” with the body between a distant background object and the Earth being called the “lensed object.”
In this case, the lensing object is a lensed galaxy cluster called SPT-CL J0615−5746 and the background objects are the Cosmic Gems, their star clusters, and two distant lensed galaxies.
“The special thing about the Cosmic Gems series is that thanks to gravitational lensing we can actually resolve the galaxy down to parsec scales!” said Adamo.
How do globular clusters form?
A promising follow-up study to this JWST observation of early star clusters looks at the formation of stellar arrangements called “globular clusters.” As can be seen in our own 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 collections of tightly packed, gravitationally bound stars come together, but crucially, massive and dense young star clusters in the Cosmic Gems arc could well be the initial stages of globular cluster formation. This means they could provide an incredibly useful window into the early stages of globular cluster formation.
These five star clusters could also contribute to understanding other aspects of cosmic evolution.
“The high density of stars in these clusters gives us the first clues about the processes taking place inside them. This gives us new insights into the possible formation of very massive stars and the nuclei of black holes, both of which are important for the evolution of galaxies,” said Adamo.
The study of the Cosmic Gems arc continues, with the team behind this research already planning to observe this early galaxy during the third operational cycle of the $10 billion space telescope using JWST’s Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI).
“The NIRSpec observations will allow us to confirm the redshift of the galaxy and to study the ultraviolet emission of the star clusters, which in turn will serve to study their physical properties in more detail,” said Bradley. “The MIRI observations will allow us to study the properties of ionized gas.”
These spectroscopic observations should provide information about how intense star formation was at the active sites of this young galaxy.
The astronomers behind this study now also intend to examine other galaxies to look for star clusters similar to these five.
“I am convinced that there are other systems like this in the early universe waiting to be discovered. This will allow us to deepen our understanding of early galaxies,” said team member Eros Vanzella of the National Institute of Astrophysics (INAF).
The team’s research was published in the journal Nature on Monday (June 24).