JWST spectroscopic confirmation of the Cosmic Gems arc at z=9.625 -- Insights into the small scale structure of a post-burst system

TL;DR

This study employs JWST/NIRSpec IFU spectroscopy of the strongly lensed Cosmic Gems arc at $z=9.625$ to confirm its redshift via a Ly$\alpha$ break and faint $H\beta$ and $[OIII]$ lines, while revealing a blue UV slope and a modest Balmer break. Spatially resolved spectroscopy identifies five compact star clusters with $R_{eff}\lesssim1$ pc and extreme stellar densities in a $\sim50$ pc region, plus four additional $\lesssim10$ pc systems along the tail, embedded in an arc of intrinsic size $\sim400$ pc; forward modeling supports very small intrinsic sizes and high magnifications ($\mu$ up to $>300$) along the critical region. Spectral energy distribution fitting with Bagpipes, combined with updated lens models, constrains a post-burst/star-formation-dormant phase for the host, with stellar ages $\sim10-20$ Myr in the clusters and a total arc stellar mass of order $\sim10^{7}-10^{8} M_\odot$. The results illustrate the power of JWST+lensing to probe sub-galactic structure and feedback in a pre-reionization galaxy, highlighting how compact star clusters can dominate the observed spectra and drive localized quenching.

Abstract

We present JWST/NIRSpec integral field spectroscopy of the Cosmic Gems arc, strongly magnified by the galaxy cluster SPT-CL J0615$-$5746. Six-hour integration using NIRSpec prism spectroscopy (resolution $\rm R\simeq 30-300$), covering the spectral range $0.8-5.3~μm$, reveals a pronounced $\rm Lyα$-continuum break at $λ\simeq 1.3~μm$, as well as weak optical $\rm Hβ$ and $\rm [OIII]\lambda4959$ emission lines at $z=9.625\pm0.002$, located in the reddest part of the spectrum ($λ> 5.1~μm$). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope ($β\leq-2.5$, $\rm F_λ \sim λ^β$). Spectral fitting with $\tt Bagpipes$ suggests that the Cosmic Gems galaxy is in a post-starburst phase, making it the highest-redshift system currently observed in a mini-quenched state. Spatially resolved spectroscopy at tens of parsecs shows relatively uniform features across subcomponents of the arc. These findings align well with the physical properties previously derived from JWST/NIRCam photometry of the stellar clusters, now corroborated by spectroscopic evidence. In particular, five observed star clusters exhibit ages of $\rm 7-30~Myr$. An updated lens model constrains the intrinsic sizes and masses of these clusters, confirming they are extremely compact and denser than typical star clusters in local star-forming galaxies. Additionally, four compact stellar systems consistent with star clusters ($\lesssim10$ pc) are identified along the extended tail of the arc. A sub-parsec line-emitting HII region straddling the critical line, lacking a NIRCam counterpart, is also serendipitously detected.

Figures (17)

• Figure 1: JWST/NIRSpec spectrum, NIRCam, and pseudo-NIRCam images (NIRSpec-IFU based) of the Cosmic Gems arc. Top: 1D NIRSpec prism spectrum (from the PSF-matched cube) extracted from the full arc (with its aperture mask shown in the bottom-central panel), showing the large Ly$\rm \alpha$-break and damping at $\simeq 1.3~\mu m$, along with two weak emission lines, $\rm H\beta$ and [O$III$1.2ex]$\lambda4959$ , on the rightmost side of the spectral range. A weak Balmer break is also detected (Section \ref{['sec:nirspec_analysis']}). The 1$\sigma$ uncertainty spectrum is shown in red. The wavelengths corresponding to the main undetected UV and optical lines are shown with dashed lines. The throughputs of the NIRCam filters covering the field are shown in green, for reference. Bottom: From left to right, JWST/NIRCam stacked F150W+F200W image with the NIRSpec-IFU field of view outlined; collapsed NIRSpec IFU data cube in the rest-UV (covering the observed wavelength range of F150W and F200W), before (middle) and after (right) PSF-matching. The last panel also includes the outlines of the masks used to extract the 1D spectra for the regions discussed in the main text.
• Figure 2: JWST/ NIRCam color image of the galaxy cluster SPT0615 field with the locations of the Cosmic Gems arc and its counter-image marked, along with the critical lines for the $z=9.625$ lens models from Lenstool (red line) and glafic (green line). The yellow-shaded square marks the field of view of the JWST/NIRSpec IFU observations.
• Figure 3: Left: 2D map of the stacked $\rm H\beta$ + [O$III$1.2ex]$\lambda4959$ in the wavelength range 5.15-5.3 $\mu$m (see text for more details). The red mask corresponds to the regions BCDE1,2 and CC (defined in Fig. \ref{['fig:nircam_nirspec']}), used to extract the spectrum on the right. The inset shows the NIRCam F150W+F200W data with overlaid black contours showing the peak of the $\rm H\beta$ + [O$III$1.2ex]$\lambda4959$ emission and the CCs from glafic (blue) and Lenstool (red). The white circle (in both the map and the inset) denote the FWHM of the NIRSpec-IFU PSF, while the NIRCam one is plotted as a black circle in the inset. Right: 1D spectrum (thick gray line) in the wavelength range of the detected emission lines. The continuum and line best-fitting result are shown as a red line. The error spectrum and spectral fit residuals are shown in green and light gray, respectively. The gray bands mark the portion of the spectrum not included in the fit.
• Figure 4: Spectrum of the combined BCDE12 regions (gray), along with the best-fit model (red), residuals (black), and $\pm1\sigma$ uncertainties (green). Bottom panels: Zoom-ins on the regions where the best-fit predicts the main emission lines.
• Figure 5: Stellar mass surface density ($\Sigma_{mass}$) and effective radii ($\rm R_{eff}$) of the star clusters discussed in this work (magenta symbols), along with the following collection of gravitationally lensed star clusters from literature: Sunburst arc vanzella2022, Sunrise arc vanzella2023_sunrise, Cosmic Archipelago Messa2025, and Firefly Sparkle mowla2024. A comparison with local YMCs from brown2021 (red contours) and Milky Way globular clusters (baumgardt2018, blue contours) shows the high surface densities in high-z clusters when compared to local ones (see also the dotted lines marking the constant stellar mass tracks). For the Cosmic Gems clusters, the black symbols refer to the values published in adamo2024a assuming $\rm z_{phot}=10.2$.