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 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 z=10.2 critical curve of our fiducial LENSTOOL-A model (see Section 4) is shown in gold. The location of the Cosmic Gems Arc is shown in the left-hand white box, with a zoomed inset figure (8?"×8?") outlining the galaxy with a red ellipse. The z=10.2 critical curves of the LENSTOOL-A (gold), LENSTOOL-B (dark orange), Glafic (cyan), and WSLAP+ (magenta) lens models (described in Section 4) bisect the Cosmic Gems Arc. The right-hand white box and zoomed inset (8?"×8?") shows the candidate counterimage of the arc, which is located near (within 2.''2) the position predicted by the lens models. 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.