TEMPLATES: Characterization of a Merger in the Dusty Lensing SPT0418–47 System

We present JWST and Atacama Large Millimeter/submillimeter Array (ALMA) imaging for the lensing system SPT0418‑47, which includes a strongly lensed, dusty, star-forming galaxy at redshift z = 4.225 and an associated multiply imaged companion. The JWST NIRCam and MIRI imaging observations presented in this paper were acquired as part of the Early Release Science program Targeting Extremely Magnified Panchromatic Lensed Arcs and Their Extended Star formation (TEMPLATES). This data set provides robust mutiwavelength detections of stellar light in both the main (SPT0418A) and companion (SPT0418B) galaxies, while the ALMA detection of [C II] emission confirms that SPT0418B lies at the same redshift as SPT0418A. We infer that the projected physical separation of the two galaxies is 4.42 ± 0.05 kpc. We derive total magnifications of μ = 29 ± 1 and μ = 4.1 ± 0.7 for SPT0418A and SPT0418B, respectively. We use both PROSPECTOR and CIGALE to derive stellar masses. We find that SPT0418A has a stellar mass of ${M}_{* }={3.4}_{-0.6}^{+1.1}\times {10}^{10}{M}_{\odot }$ from PROSPECTOR or M _* = 1.5 ± 0.3 × 10¹⁰ M _⊙ from CIGALE. The stellar mass ratio of SPT0418A and SPT0418B is roughly between 4 and 7 ( ${4.2}_{-1.6}^{+1.9}$ for PROSPECTOR and 7.5 ± 3.7 for CIGALE). We see evidence of extended structure associated with SPT0418A that is suggestive of a tidal feature. These features, along with the close projected proximity, imply that the system is interacting. Interestingly, the star formation rates and stellar masses of both galaxies are consistent with the main sequence of star-forming galaxies at this epoch, indicating that this ongoing interaction has not noticeably elevated the star formation levels.