Discovery of a Dusty, Chemically Mature Companion to a z ∼ 4 Starburst Galaxy in JWST ERS Data

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.