James Webb Space Telescope Feed Post

Unveiling the Cosmic Gems Arc at z~10.2 with JWST

Cutout images of the Cosmic Gems Arc in the observed HST and JWST filters. The cutout field of view is 4.''8 × 4.''8 and the images are shown with north up and east left. Filters are labeled in the upper-left corner of each panel with white text for HST images and gold for JWST images. The Cosmic Gems Arc extends to 5" long in the JWST NIRCam images and is centered in the cutout images. The galaxy is undetected in F115W and all bluer filters. It is weakly detected in F125W and clearly detected in F140W and all redder filters. The JWST NIRCam F150W image (PSF FWHM 0.''05) provides the best resolution of the Cosmic Gems Arc, revealing star clusters down to 1 pc in intrinsic size (Adamo et al., 2024). The nearby arc to the north has a photometric redshift of z~2.6. Abstract: We present recent JWST NIRCam imaging observations of SPT0615-JD (also known as the Cosmic Gems Arc), lensed by the galaxy cluster SPT-CL J0615-5746. The 5-arcsec-long arc is the most highly magnified z>10 galaxy known, straddling the lensing critical curve and revealing five star clusters with radii ~1 pc or less. We measure the full arc to have F200W 24.5 AB mag, consisting of two mirror images, each 25.3 AB mag with a magnification µ~60 (delensed 29.7 AB mag, MUV=-17.8). The galaxy has an extremely strong Lyman break F115W-F200W >3.2 mag (2s lower limit), is undetected in all bluer filters (<2s), and has a very blue continuum slope redward of the break (ß=-2.7±0.1), resulting in a photometric redshift zphot=10.2±0.2 (95% confidence) with no significant likelihood below z<9.8. Based on SED fitting to the total photometry, we estimate an intrinsic stellar mass of M*~2.4-5.6×107M?, young mass-weighted age of ~21-79 Myr, low dust content (AV<0.15), and a low metallicity of ?1% Z?. We identify a fainter third counterimage candidate within 2.2 arcsec of the predicted position, lensed to AB mag 28.4 and magnified by µ~2, suggesting the fold arc may only show ~60% of the galaxy. SPT0615-JD is a unique laboratory to study star clusters observed within a galaxy just 460 Myr after the Big Bang.