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

December 2022 • 2022ApJ...940L..53V

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.

Abstract • 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 μm, 1.50 μm, and 2.0 μm 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) ×10⁶ 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 (<30 Myr) inferred from the spectral energy distribution fitting and supported by the presence of high-ionization lines secured with the Very Large Telescope/MUSE. Such bursty forming regions enhance the specific star formation rate of the galaxy, which is ≃10 Gyr^-1. This galaxy would be among the extreme analogs observed in the local universe having a high star formation rate surface density and a high occurrence of massive stellar clusters in formation. *Based on observations collected with JWST under the ERS program 1324 (PI T. Treu).

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Takahiro Morishita

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