The Sunburst Arc with JWST

II. Observations of an Eta Carinae analog at z = 2.37

¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
² Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway
³ Department of Astronomy, University of Michigan, 1085 S. University Ave, Ann Arbor, MI 48109, USA
⁴ Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
⁵ Astrophysics Science Division, Code 660, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
⁶ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁷ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA
⁸ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, Texas 78712, USA
⁹ Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
¹⁰ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
¹¹ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
¹² IPAC, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA

^⋆ Corresponding author: suhyeon.choe@astro.su.se

Received: 10 May 2024
Accepted: 11 March 2025

Abstract

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 Lyβ emission, as well as Lyα-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.