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

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 $\mathrm{R}\simeq 30-300$), covering the spectral range $0.8-5.3\hat{I}¼m$, reveals a pronounced $\mathrm{Ly}\hat{\mathrm{I}}\pm$-continuum break at $\hat{I}»\simeq 1.3\hat{I}¼m$, as well as weak optical $\mathrm{H}{\hat{\mathrm{I}}}^2$ and $[\mathrm{OIII}]\lambda 4959$ emission lines at $z=9.625\pm 0.002$, located in the reddest part of the spectrum ($\hat{I}»>5.1\hat{I}¼m$). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope (${\hat{I}}^2\le -2.5$, ${\mathrm{F}}_{\hat{\mathrm{I}}}»\sim \hat{\mathrm{I}}{»}^2$). Spectral fitting with $\mathtt{Bagpipes}$ 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 $7-30\mathrm{Myr}$. 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 ($\lesssim 10$ 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.

22 pages (15 figures, 4 tables). Accepted for publication in A&A; see also the companion work Vanzella et al. 2025