JWST spectroscopic confirmation of the Cosmic Gems arc at z = 9.625Messa, Matteo (Avtor) Bradač, Maruša (Avtor) galaxiesgravitational lensingredshiftstar clustersstar formationWe 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 ▫$R \simeq 30-300$▫ covering the spectral range ▫$0.8-5.3\ \mu$▫m, covering the spectral range ▫$0.8-5.3$▫ reveals a pronounced Ly▫$\alpha$▫-continuum break at ▫$\lambda \simeq 1.3\ \mu$▫m, as well as weak optical H▫$\beta$▫ and [O III] ▫$\lambda4959$▫ emission lines at ▫$z = 9.625 \pm 0.002$▫, located in the reddest part of the spectrum (▫$\lambda>5.1\ \mu$▫m). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope (▫$\beta \leq -2.5, \text{F}_{\lambda} \sim \lambda^{\beta}$▫). 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 (▫$\Sigma_{M_{\ast}} = 10^5-10^6\ M_{\odot}$▫). 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 H II 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.20262026-04-20 12:58:30Članek v reviji181940UDK: 524.8ISSN pri članku: 1432-0746DOI: 10.1051/0004-6361/202556574COBISS_ID: 275650051sl