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arXiv · 2507.18699

The z = 9.625 Cosmic Gems Galaxy was a "Compact Blue Monster" Propelled by Massive Star Clusters

E. Vanzella·M. Messa·A. Adamo·F. Loiacono·M. Oguri·K. Sharon·L. D. Bradley·P. Bergamini·M. Meneghetti·A. Claeyssens·B. Welch·M. Bradac·A. Zanella·A. Bolamperti·F. Calura·T. Y-Y. Hsiao·E. Zackrisson·M. Ricotti·L. Christensen·J. M. Diego·F. E. Bauer·X. Xu·S. Fujimoto·C. Grillo·M. Lombardi·P. Rosati·T. Resseguier·A. Zitrin·A. Bik·J. Richard·Abdurro'uf·R. Bhatawdekar·D. Coe·B. Frye·A. K. Inoue Y. Jimenez-Teja·C. Norman·J. R. Rigby·M. Trenti·T. Hashimoto
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Abstract

The recent discovery of five massive stellar clusters at z=9.625 in the Cosmic Gems has raised the question about the formation mechanism of star clusters in the first half Gyr after the Big-Bang. We infer the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM ~ (n$_0$) $M^\beta$), assuming three different slopes $\beta$ = -1.5, -2.0 and -2.5 and different lower-mass limits between $10^2$ and $10^5$ Msun. The total integrated cluster stellar mass is compared to the stellar mass inferred from the counter-image of the Cosmic Gems, which provides the best, modestly magnified ($\mu$ = 1.84$\pm$0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy is estimated as 3.5$_{-1.8}^{+3.3}$ x$10^7$ Msun, with an effective radius of Reff = 103$_{-15}^{+13}$ parsec and a stellar surface mass density of $\Sigma$mass = 520$_{-225}^{+340}$ Msun pc$^{-2}$. Accounting for normalization uncertainties - including different lensing magnification scenarios for the arc - a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter-image, without exceeding the galaxy's stellar mass. By extrapolating the physical properties at the peak of the burst we find that in its recent past (<~ 30 Myr) the Cosmic Gems galaxy has likely experienced a specific star formation rate (sSFR) exceeding 25 Gyr$^{-1}$ and luminosity approaching the ``blue monster'' regime (M$_{UV}$ < -20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and shed light into the formation of bound star clusters that might survive to z = 0 as globular clusters, older than 13 Gyr.

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E. Vanzella, M. Messa, A. Adamo, F. Loiacono, M. Oguri, K. Sharon, L. D. Bradley, P. Bergamini, M. Meneghetti, A. Claeyssens, B. Welch, M. Bradac, A. Zanella, A. Bolamperti, F. Calura, T. Y-Y. Hsiao, E. Zackrisson, M. Ricotti, L. Christensen, J. M. Diego, F. E. Bauer, X. Xu, S. Fujimoto, C. Grillo, M. Lombardi, P. Rosati, T. Resseguier, A. Zitrin, A. Bik, J. Richard, Abdurro'uf, R. Bhatawdekar, D. Coe, B. Frye, A. K. Inoue Y. Jimenez-Teja, C. Norman, J. R. Rigby, M. Trenti, T. Hashimoto. 2025-07-24. The z = 9.625 Cosmic Gems Galaxy was a "Compact Blue Monster" Propelled by Massive Star Clusters. https://doi.org/10.1051/0004-6361%2F202556570

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