While analyzing data from the first images of a well-known early galaxy taken by NASA’s James Webb Space Telescope (JWST), Cornell University astronomers discovered a companion galaxy previously hidden behind the light of the foreground galaxy — one that surprisingly seems to have already hosted multiple generations of stars despite its young age, estimated at 1.4 billion years old.
“We found this galaxy to be super-chemically abundant, something none of us expected,” said Bo Peng, a doctoral student in astronomy, who led the data analysis. “JWST changes the way we view this system and opens up new venues to study how stars and galaxies formed in the early universe.”
Peng is the lead author of “Discovery of a Dusty, Chemically Mature Companion to z~4 Starburst Galaxy in JWST Early Release Science Data,” published in the Astrophysical Journal Letters.
Earlier images captured by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile contained hints of the companion resolved clearly by JSWT, but couldn’t be interpreted as anything more than random noise, said Amit Vishwas, a research associate at the Cornell Center for Astrophysics and Planetary Sciences (CCAPS) and the paper’s second author.
The team estimated the companion galaxy, which they labeled SPT0418-SE, was within 5 kiloparsecs of SPT0418-47, one of the brightest dusty, star-forming galaxies in the early universe, its distant light bent and magnified by a foreground galaxy’s gravity into a circle, called an Einstein ring. The Magellanic Clouds, satellites of the Milky Way are about 50 kiloparsecs away. The proximity suggests these galaxies are bound to interact with each other and potentially even merge, an observation that adds to the understanding of how early galaxies may have evolved into larger ones.
The two galaxies are modest in mass as galaxies in the early universe go, with “SE” relatively smaller and less dusty, making it appear bluer than the extremely dust-obscured ring. Based on images of nearby galaxies with similar colors, the researchers suggest that they may reside “in a massive dark-matter halo with yet-to-be-discovered neighbors.”
Most surprising about the companion galaxy, considering its age and mass, was its mature metallicity — amounts of elements heavier than helium and hydrogen, such as carbon, oxygen and nitrogen. The team estimated that as comparable to our sun, which is more than 4 billion years old and inherited most of its metals from previous generations of stars that had 8 billion years to build them up.
“We are seeing the leftovers of at least a couple of generations of stars having lived and died within the first billion years of the universe’s existence, which is not what we typically see,” Vishwas said. “We speculate that the process of forming stars in these galaxies must have been very efficient and started very early in the universe, particularly to explain the measured abundance of nitrogen relative to oxygen, as this ratio is a reliable measure of how many generations of stars have lived and died.”