Unveiling the Cosmic Gems Arc at $z\sim 10$ with JWST NIRCam

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. It straddles the lensing critical curve and reveals five star clusters with radii of $\sim 1$ 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 $\mu \sim 60_{-8}^{+17}$ (delensed 29.7 AB mag, $M_{UV}=-17.8$). The galaxy has an extremely strong Lyman break F115W$-$F200W $>3.2$ mag ($2\sigma$ lower limit), is undetected in all bluer filters ($<2\sigma$), and has a very blue continuum slope redward of the break ($\beta =-2.7\pm 0.1$). This results in a photometric redshift $z_{phot}=10.2\pm 0.2$ (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 $M_{\ast }\sim 2.4-5.6\times 10^7{M}_{\odot }$, young mass-weighted age of $\sim 21-79$ Myr, low dust content ($A_V<0.15$), and a low metallicity of $\lesssim 1\%Z_{\odot }$. We identify a fainter third counterimage candidate within 2.2 arcsec of the predicted position, lensed to AB mag 28.4 and magnified by $\mu \sim 2$, suggesting the fold arc may only show $\sim 60$% of the galaxy. SPT0615-JD is a unique laboratory to study star clusters observed within a galaxy just 460 Myr after the Big Bang.

22 pages, 8 figures, 4 tables, accepted by ApJ

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