JWSTs PEARLS: NIRCam imaging and NIRISS spectroscopy of a z = 3.6 star-forming galaxy lensed into a near-Einstein ring by a z = 1.258 massive elliptical galaxy

Abstract

We present the discovery, and initial lensing analysis, of a high-redshift galaxy–galaxy lensing system within the JWSTPEARLS/HST–TREASUREHUNT North Ecliptic Pole Time Domain Field (designated NEPJ172238.9+655143.1). The lensing geometry shears a z = 3.6 ± 0.1 star-forming galaxy into a near-Einstein ring with a radius of 0.92 arcsec, consisting of 4 images, around a foreground massive elliptical galaxy at z = 1.258 ± 0.005. The system isfortuitously located within the NIRISS F200W footprint of the PEARLS survey, enabling spectroscopic identification of the 8500 Å TiO band in the foreground galaxy and allowing tight constraints to be placed on the redshift of the background galaxy based on its continuum detection and lack of strong emission lines. We calculate magnification factors of 2.6 < μ < 8.4 for the four images and a total lensing mass of (4.08 ± 0.07) × 1011 M. SED fitting of the foreground elliptical galaxy within the Einstein radius reveals a stellar mass of 1.26 × 1011 M under a Kroupa IMF, providing 31 per cent of the estimated lensing mass. Employing simple scaling relations and assumptions, an NFW dark matter halo is found to provide the correct remaining mass within 0.12+0.21−0.09 dex. However, if a modified IMF for elliptical galaxies is employed (e.g. bottom-heavy or bottom-and-top-heavy), stellar mass estimations increase and can account for the majority of the lensing mass, reducing the need for dark matter. This system further demonstrates the new discovery space that the combined wavelength coverage, sensitivity and resolution of James Webb Space Telescope (JWST) now enables.