An international team of astronomers using the James Webb Space Telescope discovered gravitationally bound star clusters when the Universe was 460 million years old. This is the first discovery of star clusters in a young galaxy less than 500 million years after the Big Bang.
Arc of Cosmic Gems
Young galaxies in the early Universe underwent significant phases of explosive star formation, generating significant amounts of ionizing radiation. However, due to their cosmological remoteness, direct studies of their stellar content have proven difficult.
Using the Webb, an international team of astronomers has discovered five young massive star clusters in the Cosmic Gem Arc (SPT0615-JD1), a highly lensed galaxy that emitted light when the Universe was approximately 460 million years old, i.e. 97% of cosmic time.
The Cosmic Gem Arc was first discovered in Hubble Space Telescope images obtained with the RELICS (Reionization Lensing Cluster Survey) program of the lens galaxy cluster SPT-CL J0615-5746.
“It is believed that these galaxies were the main source of intense radiation that reionized the early Universe,” says lead author Angela Adamo of Stockholm University and the Oskar Klein Center in Sweden.
Authority:
ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration
Detail through gravitational lensing
The scientist notes that the peculiarity of the Cosmic Gems arc is that, thanks to gravitational lensing, it is actually possible to distinguish the galaxy down to the parsec scale!
Thanks to Webb, the science team can now see where stars are forming and how they are distributed, much like Hubble is used to study local galaxies. Webb’s view provides a unique opportunity to study star formation and the internal structure of young galaxies at such an unprecedented distance.
“The Webb’s incredible sensitivity and angular resolution in the near-infrared wavelength range, combined with gravitational lensing that provides a massive cluster of galaxies in the foreground, made this discovery possible,” explains Larry Bradley of the Space Telescope Science Institute and Webb’s science director.
“We saw a small chain of bright dots reflecting from one side to the other – these cosmic jewels are star clusters! Without Webb, we wouldn’t have known we were looking at star clusters in such a young galaxy!” adds Adamo.
Ancient globular star clusters
There are ancient globular clusters of stars in our Milky Way that are bound by gravity and have existed for billions of years. These are old relics of intense star formation in the early Universe, but it is not well understood where and when these clusters formed.
The discovery of massive young star clusters in the Cosmic Gem Arc provides scientists with an excellent insight into the early stages of a process that can develop into the formation of globular clusters. The newly discovered clusters in the arc are massive, dense, and located in a very small region of their galaxy, but they also provide most of the ultraviolet light coming from their parent galaxy.
The clusters are much denser than neighboring star clusters. This discovery will help scientists better understand how stars form in young galaxies and where globular clusters form.
The importance of discovery
The team notes that this discovery connects various scientific fields.
“These results provide direct evidence that protoglobular clusters formed in faint galaxies during the reionization era, which contributes to our understanding of how these galaxies succeeded in reionizing the Universe,” Adamo explains.
According to her, this discovery also imposes important constraints on the formation of globular clusters and their initial properties. For example, the high stellar densities found in the clusters give us the first insight into the processes occurring in their interior, which provides new knowledge about the possible formation of very massive stars and black hole seeds, which are important for the evolution of galaxies.
Research perspectives
In the future, the team hopes to create a sample of galaxies for which a similar resolution can be achieved.
“I am sure that there are other similar systems in the early Universe waiting to be discovered, allowing us to deepen our understanding of early galaxies,” said Eros Vanzella of INAF – Observatory of Astrophysics and Space Sciences in Bologna (OAS), Italy, one of the main authors of the paper.
Meanwhile, the team is preparing for further observations and spectroscopy with Webb.
“We plan to study this galaxy with the NIRSpec and MIRI instruments on Webb as part of Cycle 3,” Bradley adds.
According to him, NIRSpec observations will confirm the redshift of the galaxy and study the ultraviolet radiation of star clusters, which will be used to study their physical properties in more detail. In turn, MIRI observations will allow us to study the properties of ionized gas. The spectroscopic observations will also allow scientists to make a spatial map of the star formation rate.
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