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

Messa, Matteo (author), Bradač, Maruša (author)

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 ▫$R \simeq 30-300$▫ 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.

• Language: English
• Keywords: galaxies, gravitational lensing, redshift, star clusters, star formation
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2026
• Number of pages: 21 str.
• Numbering: Vol. 705, art. no. A173
• PID: 20.500.12556/RUL-181940
• UDC: 524.8
• ISSN on article: 1432-0746
• DOI: 10.1051/0004-6361/202556574
• COBISS.SI-ID: 275650051
• Publication date in RUL: 20.04.2026

Record is a part of a journal

• Title: Astronomy & astrophysics
• Shortened title: Astron. astrophys.
• Publisher: EDP Sciences
• ISSN: 1432-0746
• COBISS.SI-ID: 392577

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: galaksije, gravitacijsko lečenje, rdeči premik, zvezdne kopice, nastanek zvezd

Projects

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: N1-0238
• Name: Raziskave kozmične zore z James Webb vesoljskim teleskopom