Early Results from GLASS-JWST. VII. Evidence for Lensed, Gravitationally Bound Protoglobular Clusters at z = 4 in the Hubble Frontier Field A2744

• Published in: Astrophysical journal. Letters 2022-12, Vol.940 (2), p.L53
• Main Authors: Vanzella, E., Castellano, M., Bergamini, P., Treu, T., Mercurio, A., Scarlata, C., Rosati, P., Grillo, C., Acebron, A., Caminha, G. B., Nonino, M., Nanayakkara, T., Roberts-Borsani, G., Bradac, M., Wang, X., Brammer, G., Strait, V., Vulcani, B., Meštrić, U., Meneghetti, M., Calura, F., Henry, Alaina, Zanella, A., Trenti, M., Boyett, K., Morishita, T., Calabrò, A., Glazebrook, K., Marchesini, D., Birrer, S., Yang, L., Jones, T.
• Format: Article
• Language: English
• Subjects: Analogs; ASTRONOMY AND ASTROPHYSICS; Continuum radiation; Galaxies; Globular clusters; high-redshift galaxies; Hubble Space Telescope; Ionization; James Webb Space Telescope; Local group (astronomy); Massive stars; Point spread functions; Red shift; Space telescopes; Spectral energy distribution; Star & galaxy formation; Star clusters; Star formation; Star formation rate; Stellar mass; Ultraviolet astronomy; Very Large Telescope; young star clusters
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ac8c2d

We investigate the blue and optical rest-frame sizes ( λ ≃ 2300–4000 Å) of three compact star-forming regions in a galaxy at z = 4 strongly lensed (×30, ×45, and ×100) by the Hubble Frontier Field galaxy cluster A2744 using GLASS-ERS James Webb Space Telescope (JWST)/NIRISS imaging at 1.15 μ m, 1.50 μ m, and 2.0 μ m with a point-spread function ≲0.″1. In particular, the Balmer break is probed in detail for all multiply imaged sources of the system. With ages of a few tens of Myr, stellar masses in the range (0.7–4.0) ×10 6 M ⊙ and optical/ultraviolet effective radii spanning the interval 3 < R eff < 20 pc, such objects are currently the highest-redshift (spectroscopically confirmed) gravitationally bound young massive star clusters (YMCs), with stellar mass surface densities resembling those of local globular clusters. Optical (4000 Å, JWST-based) and ultraviolet (1600 Å, Hubble Space Telescope–based) sizes are fully compatible. The contribution to the ultraviolet underlying continuum emission (1600 Å) is ∼30%, which decreases by a factor of 2 in the optical for two of the YMCs (∼4000 Å rest-frame), reflecting the young ages (