[Paper Review] Unveiling the Cosmic Gems Arc at $z\sim10$ with JWST NIRCam

Larry Bradley, Angela Adamo+23 more|arXiv (Cornell University)|Apr 16, 2024

Ionosphere and magnetosphere dynamics4 citations

TL;DR

This paper presents JWST NIRCam observations of the highly magnified $z\sim10.2$ galaxy SPT0615-JD, known as the Cosmic Gems Arc, revealing five resolved star clusters at 460 Myr post-Big Bang. The galaxy exhibits a strong Lyman break, blue UV slope ($\beta = -2.7 \pm 0.1$), low dust ($A_V < 0.15$), low metallicity ($\lesssim 1\%~Z_\odot$), and young age ($\sim21-79$ Myr), suggesting it may be a low-metallicity or PopIII star-forming galaxy, making it a key probe of early galaxy formation and cosmic reionization.

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 $z&gt;10$ 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 $&gt;3.2$ mag ($2σ$ lower limit), is undetected in all bluer filters ($&lt; 2σ$), 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 $z&lt;9.8$. Based on spectral energy distribution fitting to the total photometry, we estimate an intrinsic stellar mass of $M_{*} \sim 2.4 - 5.6 imes 10^{7} M_{\odot}$, young mass-weighted age of $\sim 21 - 79$ Myr, low dust content ($A_V &lt; 0.15$), 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.

Motivation & Objective

• To study the physical properties of the most distant known lensed galaxy, SPT0615-JD, at $z\sim10.2$, using high-resolution JWST NIRCam imaging.
• To determine the intrinsic stellar mass, age, metallicity, and dust content of this high-redshift galaxy through SED fitting.
• To test whether the galaxy is a low-metallicity or Population III star-forming system based on its photometric and morphological properties.
• To assess the completeness of the observed arc by searching for additional counterimages and estimating total magnification.

Proposed method

• Acquired deep NIRCam imaging with JWST over multiple filters (F115W, F150W, F200W) to capture the Lyman break and rest-frame UV continuum.
• Used eazypy and bagpipes SED fitting codes to derive photometric redshift and physical parameters, including stellar mass, age, metallicity, and dust content.
• Performed morphological analysis to resolve the arc into five distinct star clusters with intrinsic sizes $\lesssim1$ pc.
• Applied strong gravitational lensing modeling to estimate magnification ($\mu \sim 60$ per image, total $\mu \sim 120$) and predict missing counterimage positions.
• Compared observed photometry with theoretical models to constrain the galaxy's star formation history and ionizing photon production efficiency.
• Identified a candidate third counterimage at $2\farcs2$ from the predicted position, lensed to AB mag 28.4 with $\mu \sim 2$.

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 $\sim 2\farcm 3\times 2\farcm 3$ and the image is shown with north up and east left. The $z=10.2$ critical curve of our fiducial LENSTOOL

Experimental results

Research questions

• RQ1 What is the true redshift and intrinsic physical properties of the Cosmic Gems Arc at $z\sim10.2$?
• RQ2 Are the resolved structures within the arc individual star clusters, and what are their intrinsic sizes and luminosities?
• RQ3 What is the total magnification of the galaxy, and how much of its flux is missing due to incomplete counterimage detection?
• RQ4 Does the galaxy's low metallicity and blue UV slope indicate it is a Population III or low-metallicity star-forming system?
• RQ5 What is the ionizing photon production efficiency ($\xi_{\rm ion}$) of this early galaxy, and how does it compare to typical $z\sim10$ galaxies?

Key findings

• The Cosmic Gems Arc has a photometric redshift of $z_{\rm phot} = 10.2 \pm 0.2$ (95% C.L.), with no significant likelihood below $z < 9.8$, confirming its extreme distance.
• The galaxy has a very blue UV slope of $\beta = -2.7 \pm 0.1$, indicating a young, metal-poor stellar population.
• The intrinsic stellar mass is estimated at $M_* \sim 2.4 - 5.6 \times 10^7~M_\odot$, with a mass-weighted age of $\sim 21 - 79$ Myr.
• The galaxy has a low dust content ($A_V < 0.15$) and metallicity $\lesssim 1\%~Z_$$\odot$, consistent with a primordial or Population III origin.
• Five star clusters are resolved within the arc, each with intrinsic size $\lesssim 1$ pc, marking the first such detection at $z > 10$.
• A candidate third counterimage is identified at $2\farcs2$ from the predicted position, suggesting the arc may show only $\sim60\%$ of the total flux, implying a total magnification of $\mu \sim 120$.

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 $\sim 2\farcm 3\times 2\farcm 3$ and the image is shown with north up and east left. The f