Scanning the first images of a well-known early galaxy taken by NASA’s James Webb Space Telescope (JWST), Cornell astronomers were intrigued to see a clump of light near its outer edge.
Their first focus, and the infrared observatory’s target, was SPT0418-47, one of the most luminous dusty, star-forming galaxies in the early universe, its distant light bent and magnified by the gravity of a foreground galaxy into a circle, called an Einstein ring.
But a deeper dive into the early JWST data released last fall yielded a serendipitous discovery: a companion galaxy previously hidden behind the light of the foreground galaxy, one that surprisingly appears to have already hosted several generations of stars despite its young age, estimated at 1.4. billions of years old.
“We found that this galaxy was super chemically abundant, which none of us expected,” said Bo Peng, a graduate student in astronomy, who led the data analysis. “JWST is changing the way we look at this system and opening up new arenas for studying how stars and galaxies formed in the early universe.”
Peng is lead author of “Discovery of a Dusty, Chemically Mature Companion to z~4 Starburst Galaxy in JWST Early Release Science Data,” published Feb. 17 in Astrophysical Journal Letterswith eight co-authors who are current or former members of the Department of Astronomy in the College of Arts and Sciences.
Earlier images of the same Einstein ring taken by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile contained hints of the companion resolved clearly by JSWT, but they could not be interpreted as anything more than random noise, said Amit Vishwas, a research assistant at Cornell Center for Astrophysics and Planetary Sciences (CCAPS) and the article’s second author.
By examining spectral data embedded in each pixel of images from JWST’s NIRSpec instrument, Peng identified a new light source inside the ring. He determined that the two new sources were the images of a new galaxy being gravitationally lensed by the same foreground galaxy responsible for creating the ring, although they were eight to 16 times fainter—proof of the power of JWST’s infrared vision.
Further analysis of the chemical composition of the light confirmed that strong emission lines from hydrogen, nitrogen and sulfur atoms showed similar redshifts – a measure of how much light from a galaxy extends to longer, redder wavelengths as it grows further away. That placed the two galaxies about the same distance from Earth—calculated as a redshift of about 4.2, or about 10% of the age of the Universe—and in the same neighborhood.
To confirm their discovery, the researchers returned to previous ALMA observations. They found an emission line of ionized carbon that closely matched the redshifts observed by JWST.
“It really did,” Vishwas said. “Because we have several emission lines shifted by exactly the same amount, there is no doubt that this new galaxy is where we think it is.”
The team estimated that the companion galaxy, which they labeled SPT0418-SE, was within 5 kiloparseconds of the ring. (The Magellanic Clouds, satellites of the Milky Way, are about 50 kiloparsecs away.) This proximity suggests that the galaxies are bound to interact with each other and potentially even merge, an observation that adds to the understanding of how early galaxies might have evolved to larger.
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 dusty ring. Based on images of nearby galaxies with similar colors, the researchers suggest they may reside “in a massive dark matter halo with neighbors yet to be discovered.”
Most surprising about the companion galaxy, given 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 it 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 see the remnants of at least a couple of generations of stars that lived and died during the first billion years of the universe’s existence, which is not what we usually 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, especially to explain the measured abundance of nitrogen to oxygen, since this ratio is a reliable measure of how many generations stars have lived and died.”
The researchers have submitted a proposal for JWST observing time to continue the study of the ring and its companions and reconcile potential differences observed between the optical and far-infrared spectra.
“We are still working on this galaxy,” Peng said. “There is more to explore in this data.”
The team thanked the early release science program that made the JWST data immediately available to the public, called TEMPLATES: Targeting Extremely Magnified Panchromatic Lensed Arcs and Their Extended Star formation, led by NASA astrophysicist Jane Rigby, the observatory’s operations project scientist.
Bo Peng et al, Discovery of a Dusty, Chemically Mature Companion to az ~ 4 Starburst Galaxy in JWST ERS Data, The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acb59c
Astrophysical Journal Letters