Unveiling the Cosmic Gems Arc at $z\sim10$ with JWST NIRCam

TL;DR

The study targets the Cosmic Gems Arc (SPT0615-JD) lensed by SPT-CL J0615-5746 to investigate star formation in a galaxy formed within the first 470 Myr of the Universe. By combining JWST/NIRCam imaging with strong gravitational lensing, the authors resolve five young star clusters with radii of about $\sim 1\,\mathrm{pc}$ and quantify the arc's photometric properties, including $F200W=24.5$ AB mag and a mean magnification $\mu \sim 60^{+17}_{-8}$ (delensed $M_{UV}=-17.8$). Photometric redshift is constrained to $z_{\mathrm{phot}} = 10.2 \pm 0.2$ (95% CL), with a pronounced Lyman break ($F115W-F200W > 3.2$ mag) and a blue continuum ($\beta = -2.7 \pm 0.1$); SED fitting yields an intrinsic stellar mass $M_{*} \sim 2.4{-}5.6 \times 10^{7} M_{\odot}$, a mass-weighted age of $\sim 21{-}79$ Myr, low dust content ($A_V < 0.15$), and metallicity $\lesssim 1\%\,Z_{\odot}$. A fainter counterimage near predicted position (AB mag 28.4, $\mu \sim 2$) within $2.2^{\prime\prime}$ suggests the fold arc reveals only $\sim 60\%$ of the galaxy. Collectively, SPT0615-JD provides a unique, spatially resolved laboratory to study star clusters in a galaxy observed only $\sim 460$ Myr after the Big Bang.

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 $μ\sim 60^{+17}_{-8}$ (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 ($β= -2.7 \pm 0.1$). 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 $M_{*} \sim 2.4 - 5.6 \times 10^{7} M_{\odot}$, 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 $μ\sim 2$, 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.

Figures (2)

• Figure 1: 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 $\sim 2\hbox{$.\mkern-4mu^\prime$}3 \times 2\hbox{$.\mkern-4mu^\prime$}3$ 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 \ref{['sec:lens_models']}) is shown in gold. The location of the Cosmic Gems Arc is shown in the left-hand open white box, with a zoomed inset figure ($8\hbox{$^{\prime\prime}$} \times 8\hbox{$^{\prime\prime}$}$) 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 \ref{['sec:lens_models']}) bisect the Cosmic Gems Arc. The right-hand white box and zoomed inset ($8\hbox{$^{\prime\prime}$} \times 8\hbox{$^{\prime\prime}$}$) shows the candidate counterimage of the arc, which is located near (within 2$.\!\!^{\prime\prime}$2) the position predicted by the lens models.
• Figure 2: JWST NIRCam color image of the SPT-CL J0615$-$5746 parallel field, located north-northwest of the cluster field (red: F115W + F150W, green: F200W + F277W, blue: F356W + F444W). The field of view is $\sim 2\hbox{$.\mkern-4mu^\prime$}3 \times 2\hbox{$.\mkern-4mu^\prime$}3$ and the image is shown with north up and east left. The field partially includes ESO 121$-$20 (outlined in red), an isolated dwarf irregular galaxy at a distance of 6.05 Mpc Karachentsev2006.