2026A&A...705A.173M

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 R ≃ 30 - 300), covering the spectral range 0.8 - 5.3 μm, reveals a pronounced Lyα-continuum break at λ ≃ 1.3 μm, as well as weak optical Hβ and [O III] λ4959 emission lines at z = 9.625 ± 0.002, 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 ≤ -2.5, F_λ ∼ λ^β^). Spectral fitting with 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 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 (Σ_M★ = 10⁵ - 10⁶ M_⊙). Additionally, four compact stellar systems consistent with star clusters (≲10 pc) are identified along the extended tail of the arc. A sub-parsec line-emitting HII1.2ex region straddling the critical line, lacking a NIRCam counterpart, is also serendipitously detected. The Cosmic Gems arc thus offers a rare opportunity to investigate, at parsec scales, the aftermath of a star formation burst in the early Universe.