Finally spotted! JWST caught a 1.4 billion year old, metal-rich galaxy

Analyzing the data of the first image captured by NASA’s JWST (James Webb Space Telescope) of a popular early galaxy, astronomers at Cornell University were surprised by the blob of light shining near the galaxy’s outer edge.   

While scanning the image, the initial focus and target of the infrared observatory was SPT0418-47, one of the brightest dusty, star-creating galaxies in the early universe. 

Its distant light bent and magnified into a circle (Einstein ring) by the gravity of a foreground galaxy.   

However, when the astronomers took a deeper look into previous JWST data, a serendipitous discovery was produced: a companion galaxy previously overshadowed by the light of the foreground galaxy. 

As per estimates made by researchers, the galaxy is about 1.4 billion years old. Despite its young age, the galaxy has surprisingly hosted multiple generations of stars.   

Metal-rich galaxies in the early universe

“We found this galaxy to be super-chemically abundant, something none of us expected,” said Bo Peng in a press release, who is a doctoral student in astronomy and 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.” 

The paper’s second author, Amit Vishwas said earlier images captured by the Atacama Large Millimeter/submillimeter Array (ALMA) of the same galaxy hinted towards the companion galaxy, which JSWT confirmed. However, nothing more than random noises could be interpreted.   

Considering their mass and age, the most surprising feature of these two galaxies was their mature metallicity—amounts of elements like carbon, oxygen and nitrogen that are heavier than Hydrogen and Helium —which is quite similar to the sun.     

"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 explained.

"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." 

“We’re still working on this galaxy,” Peng said. “There’s more to explore in this data.” 

The full study was published in the Astrophysical Journal Letters and can be found here.

Study abstract

We report the discovery of two companion sources to a strongly lensed galaxy SPT0418-47 ("ring") at redshift 4.225, targeted by the JWST Early Release Science program. 

We confirm that these sources are at a similar redshift to the ring based on Hα detected in the NIRSpec spectrum and [C ii] λ158 μm line from the Atacama Large Millimeter/submillimeter Array (ALMA). 

Using multiple spectral lines detected in JWST/NIRSpec, the rest-frame optical to infrared images from NIRCam and MIRI and far-infrared dust continuum detected by ALMA, we argue that the newly discovered sources are actually lensed images of the same companion galaxy SPT0418-SE, hereafter referred to "SE," located within 5 kpc in the source plane of the ring. 

The star formation rate derived using [C ii] and the dust continuum puts a lower limit of 17 M☉ yr−1, while the SFRHα is estimated to be >2 times lower, thereby confirming that SE is a dust-obscured star-forming galaxy. Analysis using optical strong line diagnostics suggests that SE has near-solar elemental abundance, while the ring appears to have supersolar metallicity O/H and N/O. 

We attempt to reconcile the high metallicity in this system by invoking early onset of star formation with continuous high star-forming efficiency or by suggesting that optical strong line diagnostics need revision at high redshift.

We suggest that SPT0418-47 resides in a massive dark-matter halo with yet-to-be-discovered neighbors. This work highlights the importance of joint analysis of JWST and ALMA data for a deep and complete picture of the early universe.

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