The Sunburst Arc with JWST

Context. The peculiar object known as Godzilla resides within the gravitationally lensed Sunburst Arc at z = 2.37. Despite being very bright, it appears in only one of the twelve lensed images of the source galaxy, exhibiting unusual spectroscopic properties that have not been found in any other clumps. Aims. We use JWST’s unique combination of spatial resolution and spectroscopic sensitivity to propose a unified, coherent explanation of the physical nature of Godzilla. Methods. We measured the fluxes and kinematic properties of rest-optical emission lines in Godzilla and its surrounding regions. Using standard line ratio-based diagnostic methods in combination with NIRCam imaging and ground based rest-UV spectra, we have characterized Godzilla and its surroundings. Results. Among the set of around 60 detected lines, we found a cascade of strong O I lines pumped by intense Ly β emission, as well as Ly α -pumped rest-optical Fe II lines, reminiscent of the Weigelt blobs in the local luminous blue variable (LBV) star Eta Carinae. The spectra and imagery of Godzilla and two faint adjacent images, along with the detection of a low-surface brightness foreground galaxy in the NIRCam data support the interpretation that Godzilla is an extremely magnified object, due to the alignment with lensing caustics. We find that Godzilla is part of a previously identified clump, comprising ∼10 − 25 % of it, with a magnification in the range of ≈600 − 25 000 (depending on the models and images used for comparison). The unique O I source in Godzilla is aptly explained as a non-erupting LBV accompanied by a hotter companion and/or gas condensations exposed to more intense radiation, compared to the Weigelt blobs. If Godzilla is confirmed to contain an LBV star in future studies, this finding would expand the distance to the furthest known LBV from a dozen Mpc to several Gpc.