Discovery

JWST-ER1 is located in the COSMOS-Web JWST data¹, as described in the main text. We reduced and aligned the NIRCam images with a software pipeline that was previously developed for the Hubble Space Telescope (HST) imaging and was modified for the JWST instruments⁴⁷. Existing HST/ACS F814W imaging from the original COSMOS project⁴⁸ and datasets at other wavelengths were processed in the same way, so that all space-based images are aligned to a common astrometric frame. The galaxy was found from a visual inspection of a mosaic that was generated from the F115W, F277W and F444W data.

It is not the first Einstein ring that was found in the COSMOS field; there are at least two others, along with several more candidates⁴⁹. This raises the question of why JWST-ER1 had not been noticed before. The main reason is that both the source and the lens are faint in the optical, and that existing HST data in the near-infrared—while showing the lens—are not deep enough to show the source. In Supplementary Fig. 1 the pre-JWST high-resolution data are shown: the HST/ACS F814W from the original COSMOS programme⁴⁸ and a short-exposure HST/WFC3 F160W image from 3D-DASH, a wide-field survey with the drift-and-shift (DASH) technique⁵⁰. With the benefit of hindsight, the characteristics of an Einstein ring can be glimpsed: a compact red galaxy near the centre of a blue ring.