Unveiling the Cosmic Gems Arc at z∼10.2 with JWST

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, M_UV = -17.8). The galaxy has an extremely strong Lyman break F115W-F200W >3.2 mag (2σ lower limit), is undetected in all bluer filters (< 2σ), and has a very blue continuum slope redward of the break (β = -2.7 ± 0.1), resulting in a photometric redshift z_phot = 10.2 ± 0.2 (95 z < 9.8. Based on SED fitting to the total photometry, we estimate an intrinsic stellar mass of M_*∼ 2.4 - 5.6 × 10^7 M_⊙, young mass-weighted age of ∼ 21 - 79 Myr, low dust content (A_V < 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 clusters observed within a galaxy just 460 Myr after the Big Bang.