The newly-discovered galaxy, SPT0418-SE, and its much larger companion, SPT0418-47, reside in a massive dark-matter halo with yet-to-be-discovered neighbors, according to a new study.
SPT0418-47 is gravitationally lensed by a foreground galaxy, appearing in the sky as a near-perfect ring of light — a so-called Einstein ring. Image credit: ALMA / ESO / NAOJ / NRAO / Rizzo et al.
Scanning the first images of SPT0418-47, a well-known strongly-lensed galaxy taken by the NASA/ESA/CSA James Webb Space Telescope, Cornell University astronomer Bo Peng and colleagues were intrigued to see a blob of light near its outer edge.
SPT0418-47’s light was bent and magnified by a foreground galaxy’s gravity into a circle, called an Einstein ring.
A deeper dive into the early Webb data released last fall produced a serendipitous discovery: 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,” Peng said.
“Webb changes the way we view this system and opens up new venues to study how stars and galaxies formed in the early Universe.”
“Earlier images of the same Einstein ring captured by the Atacama Large Millimeter/submillimeter Array (ALMA) contained hints of the companion resolved clearly by JSWT, but they couldn’t be interpreted as anything more than random noise,” said Dr. Amit Vishwas, an astronomer at Cornell University.
Investigating spectral data embedded in each pixel of images from Webb’s NIRSpec instrument, the astronomers identified a second new light source inside the ring.
They 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.
Further analysis of the light’s chemical composition confirmed that strong emission lines from hydrogen, nitrogen and sulfur atoms displayed similar redshifts — a measure of how much light from a galaxy stretches into longer, redder wavelengths as it grows more distant.
That placed the two galaxies roughly the same distance from Earth — calculated as a redshift of about 4.2, or about 10% of the Universe’s age — and in the same neighborhood.
To verify their discovery, the researchers returned to earlier ALMA observations.
They found an emission line of ionized carbon closely matched the redshifts observed by Webb.
They estimated the companion galaxy, SPT0418-SE, was within 16,300 light-years (5,000 parsecs) of the ring.
That proximity suggests the 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 SPT0418-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 authors suggest that they may reside in a massive dark-matter halo with yet-to-be-discovered neighbors.
Most surprising about these galaxies, considering their age and mass, was their mature metallicity — amounts of elements heavier than helium and hydrogen, such as carbon, oxygen and nitrogen — which the team estimated to be similar to our Sun.
Compared to the Sun, which is about 4 billion years old and inherited most of its metals from previous generations of stars that had roughly 8 billion years to build them up, we are observing these galaxies at a time when the Universe was less than 1.5 billion years old.
“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,” Dr. 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.”
The paper was published in the Astrophysical Journal Letters.
_____
Bo Peng et al. 2023. Discovery of a Dusty, Chemically Mature Companion to a z ~ 4 Starburst Galaxy in JWST ERS Data. ApJL 944, L36; doi: 10.3847/2041-8213/acb59c
