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

Journal
Date Issued
2022
Author(s)
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Bergamini, P.
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Treu, T.
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Scarlata, C.
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Rosati, P.
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Grillo, C.
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Acebron, A.
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Caminha, G. B.
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Nanayakkara, T.
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Roberts-Borsani, G.
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Bradac, M.
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Wang, X.
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Brammer, G.
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Strait, V.
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Henry, Alaina
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Trenti, M.
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Boyett, K.
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Morishita, T.
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Calabrò, A.
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Glazebrook, K.
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Marchesini, D.
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Birrer, S.
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Yang, L.
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Jones, T.
DOI
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)....
Volume
940
Issue
2
Start page
L53
Uri
Url
Issn Identifier
2041-8205
Ads BibCode
2022ApJ...940L..53V
Rights
open.access
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