The z = 9.625 Cosmic Gems galaxy was a compact “blue monster” propelled by massive star clusters

Vanzella, Eros (author), Bradač, Maruša (author)

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, ▫$\text{d}n(M)/\text{d}M = n_0 M^{\beta}$▫), assuming three different slopes (▫$\beta = -1.5, -2.0$▫, and ▫$-2.5$▫) and different lower-mass limits between ▫$10^2$▫ and ▫$10^5$▫ ▫$\text{M}_{\odot}$▫. 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 (▫$\mu = 1.84 \pm 0.05$▫) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy was estimated as ▫$3.5_{-1.8}^{+3.3}\times10^7\ \text{M}_{\odot}$▫, with an effective radius of ▫$R_{\text{eff}} = 103_{-15}^{+13}$▫ parsecs and a stellar surface mass density of ▫$\Sigma_{\text{mass}} = 520_{-225}^{+340}\ \text{M}_{\odot}\ \text{pc}^{-2}$▫. 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 (▫$\lesssim$▫30 Myr) the Cosmic Gems galaxy likely experienced a specific star formation rate exceeding ▫$25\ \text{Gyr}^{-1}$▫ and luminosity approaching the “blue monster” regime (▫$M_{\text{UV}} < -20$▫). 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.

• Language: English
• Keywords: galaxies, high-redshift, 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: 11 str.
• Numbering: Vol. 705, art. no. A171
• PID: 20.500.12556/RUL-181939
• UDC: 524.8
• ISSN on article: 1432-0746
• DOI: 10.1051/0004-6361/202556570
• COBISS.SI-ID: 275763971
• 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, rdeči premik, nastanek zvezd