It was just an intriguing and unusual point of light in the field of view of the Hubble Space Telescope, but this simple observation set the stage for a major discovery by the James Webb Space Telescope (JWST).
The complex shape of Gz9p3 indicates that it results from the merger of several galaxies. Direct imaging by the JWST reveals that Gz9p3 has a double nucleus, a sign of an ongoing merger.
Credit: NASA/Boyett et al
James Webb, Hubble's successor, has unveiled the existence of Gz9p3, one of the oldest galaxies ever identified, seen as it was just 510 million years after the Big Bang. At that time, the Universe was still in its early youth, far from its current age of 13.8 billion years.
What is striking to researchers is that Gz9p3, despite the presumed immaturity of the Universe at this period, seems to contain several billion stars and has a mass much greater than anticipated for a galaxy from that era. Its mass is about ten times that of other galaxies observed by the JWST at comparable times in the Universe's history.
The mystery deepens with the very structure of Gz9p3, which provides clues about its formation. Thanks to direct imaging by the JWST, scientists have discovered that Gz9p3 has a complex shape with two dense nuclei, suggesting that it is the result of a merger between two primitive galaxies. This merger might even be still ongoing.
The study of Gz9p3's stellar population reveals an abundance of ancient stars that survived supernovae explosions, which enriched the early Universe with heavy metals like silicon, carbon, and iron. This process played a key role in the construction of subsequent generations of stars.
The ability of galaxies to rapidly accumulate mass and form stars in the early Universe challenges our current understanding. The case of Gz9p3 shows that galaxies could become "chemically mature" much more quickly than previously thought, raising new questions about the speed at which the Universe was structured after the Big Bang.