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

Creators: Vanzella, E.¹; Castellano, M.²; Bergamini, P.^{3, 1}; Treu, T.⁴; Mercurio, A.⁵; Scarlata, C.⁶; Rosati, P.^{7, 1}; Grillo, C.^{3, 8}; Acebron, A.^{3, 8}; Caminha, G. B.⁹; Nonino, M.¹⁰; Nanayakkara, T.^{11, 12}; Roberts-Borsani, G.⁴; Bradac, M.^{13, 14}; Wang, X.^{15, 16}; Brammer, G.¹⁷; Strait, V.¹⁷; Vulcani, B.¹⁸; Meštrić, U.¹; Meneghetti, M.¹; Calura, F.¹; Henry, Alaina^{19, 20}; Zanella, A.¹⁸; Trenti, M.^{21, 12}; Boyett, K.^{21, 12}; Morishita, T.^{15, 16}; Calabrò, A.²; Glazebrook, K.¹¹; Marchesini, D.²²; Birrer, S.^{23, 24, 25}; Yang, L.²⁶; Jones, T.¹⁴

1. Osservatorio di Astrofisica e Scienza dello Spazio
2. Astronomical Observatory of Rome
3. University of Milan
4. University of California, Los Angeles
5. Astronomical Observatory of Capodimonte
6. University of Minnesota
7. University of Ferrara
8. Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano
9. Max Planck Institute for Astrophysics
10. Trieste Astronomical Observatory
11. Swinburne University of Technology
12. Centre of Excellence for All-Sky Astrophysics
13. University of Ljubljana
14. University of California, Davis
15. Infrared Processing and Analysis Center
16. California Institute of Technology
17. University of Copenhagen
18. Osservatorio Astronomico di Padova
19. Space Telescope Science Institute
20. Johns Hopkins University
21. University of Melbourne
22. Tufts University
23. Kavli Institute for Particle Astrophysics and Cosmology
24. SLAC National Accelerator Laboratory
25. Stony Brook University
26. Kavli Institute for the Physics and Mathematics of the Universe

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.

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Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank the referee for the very fast and constructive report. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program JWST-ERS-1342. We acknowledge financial support from NASA through grant JWST-ERS-1324. K.G. and T.N. acknowledge support from Australian Research Council Laureate Fellowship FL180100060. M.B. acknowledges support by the Slovenian national research agency ARRS through grant N1-0238. We acknowledge financial contributions by PRIN-MIUR 2017WSCC32 "Zooming into dark matter and proto-galaxies with massive lensing clusters" (PI: P. Rosati) and 2020SKSTHZ, INAF "main-stream" 1.05.01.86.20: "Deep and wide view of galaxy clusters" (PI: M. Nonino) and INAF "main-stream" 1.05.01.86.31 "The deepest view of high-redshift galaxies and globular cluster precursors in the early Universe" (PI: E. Vanzella).

Facilities

JWST(NIRISS) - , HST(ACS/WFC3) - , VLT(MUSE) - .

Additional Information

Based on observations collected with JWST under the ERS program 1324 (PI T. Treu).

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