JWST spectroscopic confirmation of the Cosmic Gems arc at z=9.625 -- Insights into the small scale structure of a post-burst system
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2025
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| author | Messa, M. Vanzella, E. Loiacono, F. Adamo, A. Oguri, M. Sharon, K. Bradley, L. D. Christensen, L. Claeyssens, A. Richard, J. Abdurro'uf Bauer, F. E. Bergamini, P. Bolamperti, A. Bradač, M. Calura, F. Coe, D. Diego, J. M. Grillo, C. Hsiao, T. Y-Y. Inoue, A. K. Fujimoto, S. Lombardi, M. Meneghetti, M. Resseguier, T. Ricotti, M. Rosati, P. Welch, B. Windhorst, R. A. Xu, X. Zackrisson, E. Zanella, A. Zitrin, A. |
| author_facet | Messa, M. Vanzella, E. Loiacono, F. Adamo, A. Oguri, M. Sharon, K. Bradley, L. D. Christensen, L. Claeyssens, A. Richard, J. Abdurro'uf Bauer, F. E. Bergamini, P. Bolamperti, A. Bradač, M. Calura, F. Coe, D. Diego, J. M. Grillo, C. Hsiao, T. Y-Y. Inoue, A. K. Fujimoto, S. Lombardi, M. Meneghetti, M. Resseguier, T. Ricotti, M. Rosati, P. Welch, B. Windhorst, R. A. Xu, X. Zackrisson, E. Zanella, A. Zitrin, A. |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_18705 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | JWST spectroscopic confirmation of the Cosmic Gems arc at z=9.625 -- Insights into the small scale structure of a post-burst system Messa, M. Vanzella, E. Loiacono, F. Adamo, A. Oguri, M. Sharon, K. Bradley, L. D. Christensen, L. Claeyssens, A. Richard, J. Abdurro'uf Bauer, F. E. Bergamini, P. Bolamperti, A. Bradač, M. Calura, F. Coe, D. Diego, J. M. Grillo, C. Hsiao, T. Y-Y. Inoue, A. K. Fujimoto, S. Lombardi, M. Meneghetti, M. Resseguier, T. Ricotti, M. Rosati, P. Welch, B. Windhorst, R. A. Xu, X. Zackrisson, E. Zanella, A. Zitrin, A. Astrophysics of Galaxies 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. |
| title | JWST spectroscopic confirmation of the Cosmic Gems arc at z=9.625 -- Insights into the small scale structure of a post-burst system |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2507.18705 |