James Webb Space Telescope Feed Post

Unveiling the Cosmic Gems Arc at z~10.2 with JWST

JWST NIRCam color image of the SPT-CL J0615-5746 parallel field, located north-northwest of the cluster field (red: F115W + F150W, green: F200W + F277W, blue: F356W + F444W). The field of view is ~2?.'?3×2?.'?3 and the image is shown with north up and east left. The field partially includes ESO 121-20 (outlined in red), an isolated dwarf irregular galaxy at a distance of 6.05 Mpc (Karachentsev et al., 2006). 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.