James Webb Space Telescope Feed Post

Unveiling the Cosmic Gems Arc at z~10.2 with JWST

Cutout images of the Cosmic Gems Arc (top) and the candidate counterimage (bottom), showing the Lyman-a-break using the JWST bands only. The field of view of the cutouts is 5?"×6?" and the images are shown with north up and east left. The stacked images blueward (F090W + F115W) and redward (F150W + F200W + F356W + F410M + F444W) of the Lyman-a-break are shown in the left and central panels, respectively. The right panels show color composites in the NIRCam filters. The Cosmic Gems Arc has an extremely strong NIRCam F115W-F200W break of >3.2 mag (2?o lower limit), is undetected (<2?o) in all bluer filters, and has a very blue continuum slope redward of the break. The candidate counterimage has similar colors to the arc, but is 3.9 mag fainter, with an observed F200W magnitude of oAB=28.4, fully consistent with the LENSTOOL-A model prediction of the counterimage being ~3.7-4.7 mag fainter. The predicted locations of the counterimage from the LENSTOOL-A and Glafic models (star symbols in the bottom-center panel) are within 1.''8 and 2.''2 of the candidate counterimage, respectively. 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.