2025ApJ...991...32B

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″ long arc is the most highly magnified z > 10 galaxy known. It straddles the lensing critical curve and reveals five star clusters with radii of ∼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 median magnification of $\mu \sim 6{0}_{-8}^{+17}$ (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). This results in a photometric redshift z_phot = 10.2 ± 0.2 (95% confidence) with no significant likelihood below z < 9.8. Based on spectral energy distribution 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 (A_V < 0.15), and a low metallicity of ≲1% Z_⊙. We identify a fainter third counterimage candidate within 2$\mathop{.}\limits{^{\prime\prime} }$2 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.