2026A&A...705A.171V

The recent discovery of five massive stellar clusters at z = 9.625 in the Cosmic Gems galaxy has raised the question about the formation mechanism of star clusters in the first 500 Myr after the Big Bang. We inferred the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM = n₀M^β^), assuming three different slopes (β = -1.5, -2.0, and -2.5) and different lower-mass limits between 10² and 10⁵ M_⊙. We compared the total integrated cluster stellar mass to the stellar mass inferred from the counter image of the Cosmic Gems, which provides the best modestly magnified (μ = 1.84 ± 0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy was estimated as $ 3.5_{-1.8}^{+3.3}\times 10^7 $ M_⊙, with an effective radius of $ R_{\mathrm{eff}} = 103_{-15}^{+13} $ parsecs and a stellar surface mass density of $ \Sigma_{\mathrm{mass}} = 520_{-225}^{+340} $ M_⊙ 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 found that in its recent past (≲30 Myr) the Cosmic Gems galaxy likely experienced a specific star formation rate 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 sheds light on the formation of bound star clusters that might survive to z = 0 as globular clusters older than 13 Gyr.